Honestly, plain old ignorance. (and some anglo-centrism)

I am a software dev, worked on two translation projects at different points in time, and both of them were kind of a mess. In one case, translation team was all Americans (US company), and I was the only person who spoke another language and had firsthand experience with bad translation in media. When I asked how to switch the language in their app, senior dev told me to switch my OS language. Translations themselves often sounded overly verbose, robotic, or plain weird in other languages.

And then, the typical oversights like not leaving enough screen space for longer translated text, using ambiguous terms without providing context, badly splitting phrases. Text-in-image, etc.