Translation is a funny business... I've been working in this field now for about fifteen years, and I enjoy translating from Korean to English. However, I avoid, at almost all cost, proofreading, editing and performing other quality-assurance tasks of Korean to English translations done by others. (BTW, this does not apply to doing QA on the work of my teams for English to Korean translation, which I get involved in deeply.)
Reason #1 - Rates are too low.
These additional tasks are generally billed at hourly rates. But on translation, I can charge per-word rates, and I'm a fast, efficient worker. In the calculation of hourly rates, the market doesn't take this into account; clients look at the dollar value first and last, meaning that I can make nearly twice as much on translation as on other tasks. Because this hourly rate is so symbolic of one's overall value in what's, unfortunately, a semi-commoditized business, I'm forced to quote unremarkable hourly rates in order to avoid the stigma of charging above the average.
Reason #2 - Schedules are inconvenient.
Proofreading, editing and Q/A are often regarded as afterthoughts, to be scheduled in around the really important tasks. This means that the turnarounds on these jobs are usually rushed, and if the translator gets behind schedule, it's the proofreader who has to accommodate.
Reason #3 - I'm burdened with final responsibility for the project.
Because clients have often chosen their translators based on lower rates (or just easier availability), the work I get to check is a mixed bag. This is especially true if the client was thinking they could use a cheap non-native English-speaker translator on the expectation that their proofreader would fix everything. (This happens in the Korean>English language pair a lot!) Once a project reaches me, the client wants to get back something perfect.
Reason #4 - I have to think and work outside my comfort zone.
I find that editing, proofreading and Q/A work requires more mental flexibility. On translation, I start with a clean slate and can just translate through with my own style. When working on what others have produced, it can be hard to make corrections and revisions in a consistent way, which creates a lot of thought dissonance and slows me down. Perhaps it would be different if I were given better translations to check or if I got more used to this type of work, but I find proofreading, editing and QA unpleasant compared with just doing the translation myself from the beginning.
Reason #5 - I am responsible for a budget without knowing how much effort the job will take.
It's very hard to know in advance how much time and energy investment these jobs will take, but hourly rates still come with both explicit and implicit budgets. As explained above, I end up checking work at various levels of quality. Make too many changes, and the costs go up (the client isn't happy) or make too few changes and the client wants to know why I didn't fix this or that (the client still isn't happy). If I manage to charge a per-word rate on the job, I might still get taken to the cleaners if the original translation is worse than I expected and requires a lot of extra work time that I'm not paid for. And if an hourly job takes less time than expected, then the client gets the full benefit from this and I'm left with an unpaid hole in my schedule.
Reason #6 - I have to deal with offended translators.
Clients frequently forward a proofread document back to the translator for comments, review, approval, or reflection -- or even to make the translator take notice of the errors so that he or she will do better next time. I tend to make a lot of changes since I take Reason #3 seriously, and a translator can get defensive if feeling unjustly criticized. This can lead to emotionally exhausting and time-consuming follow-up discussions that are, of course, not included in the job price.
Reason #7 - I become the go-to person for all project matters.
Perhaps it's because I'm at my computer all the time and am easy to reach, or because I can comment with more confidence about various translation issues due to my long experience, but I find that after I deliver (regardless of whether it's a translation or a proofreading/editing or Q/A), the customer makes me the main point of contact for the work through to the end. These requests are generally just assumed to be free, and if I was already underpaid for the work (Reason #1), it's hard to get excited about further distractions from other projects I am working on.
Reason #8 - The standards are higher for proofreading, editing and Q/A.
I try really hard to deliver good work, but an isolated typo, missed text or mistranslation isn't the end of the world at the translation step. However, if I, as the proofreader, miss one of these in someone else's translation, bad things happen. Unfortunately, if the proofreading or another quality assurance task involves a lot of changes, then mistakes get missed and new errors creep in and that ends up reflecting badly on me. And if there are a lot of fixes on a post-layout proof, I'll often get asked to check again (for free!) to "make sure it's good now."
Reason #9 - Many of these projects tend to be more complicated than usual and this can lead to inadvertent errors.
Many times, an old source document has been updated and so the translation based on the original also needs to be updated to match. But calculating the per-word rate for this is tough, since the changes are interspersed in the document. Therefore, hourly billing is the norm. In addition to the (unfavorable) hourly rates, updating a translation based on Tracked Changes in an old MS Word document is complicated and unpleasant. Not only that, with pressure to stay within whatever budget the client thinks the work should take, I need to move through quickly. In an already complicated updating situation, this leads to mistakes. And mistakes... lead to unhappy clients and free follow-up support.
Reason #10 - Clients rarely proofread my translation work, making translation even more attractive by comparison.
Perhaps it's because I do such a good job (^^ pat on back ^^) or maybe it's just that everyone else in our business hates proofreading, editing and Q/A work too, but for the Korean > English translations that I handle, it's rare that a client adds an editor or proofreader to the process. It can be a problem if they do, since if they send the other linguist's review of my work back and want me to go through and prepare a final version, they usually expect this to be free but I want to charge. Mercifully though, this seldom happens and so once I deliver a job that I've translated myself, I'm almost always home-free after delivery.
For all these reasons, I find it is good practice to respond to proofreading, editing and Q/A job requests with "Unfortunately, I'm very busy this week on other projects. I'm sorry for being unavailable this time."