I feel like one reason is that subtitles in Japan never match what the characters actually say. A character may say "I've missed you so much. It's been so long." The subtitles will read "Hey. Long time. " (both quotes would be Japanese) Not sure why but the Japanese subtitle industry is just terrible in so many ways.
Funnily enough, the complaints you have here actually apply more to English subtitles in my experience. Modern English subtitles tend to accurately transcribe what was said, but if you look at official subtitles from the mid-2000s and earlier you'll see that most subtitles are made much shorter than the original text. Tom Scott's video on subtitling talks about this historical practice and how it is different today[1].
Are you sure that the subtitle services you used are actually using official subtitles, and that they aren't actually translations or from some other source? How old are the shows you're watching (I believe some movies from the 70s I watched were character-accurate but that might not have been as common in the past)?
This is standard in closed captions and is not specific to Japan. So perhaps the service you're talking about only has closed captions and is incorrectly marking them as subtitles.