I think it is because the speed which subtitles need to be shown at if it would be word for word. It would be too fast for many viewers. Usually (I am talking from a Swedish context), they are a 60-75% summary of what is being said.
Super common in French, and was a big hurdle in learning the language through movies/ TV.
I don’t want to put English subtitles, but sometimes I have trouble with accents. My best guess why would be that they are translated independently and at different times by different people. Some shows almost every single line is different.
