I think this is the characteristic of the "subtitle community".
Historically, before American (or western, but mostly American) shows are widely legally imported to China, the Chinese audience pretty much relied on piracy as their only source.
The subtitles of those, are of course contributed by the community, which, by its origin, is of "various" quality. This is still the case now because some publishers would purchase subtitles from the community and pair it with legally imported shows.
So, to directly answer your question: because most of the time they are not created by professionals.