TL;DR:
Modern actors are incompetent self entitled imbeciles with no theatre training whatsoever who mumble their words because they think it sounds cooler.
Directors are incompetent self entitled imbeciles who believe unintelligible dialogue is more realistic, but also that drowning it in way too loud music makes the film more emotional, that sound recording equipment gets in the way of filming and should be kept as far away as possible from the action, and that if they know what the script says (because they've read it a thousand times) so will the audience.
Producers (and the aforementioned actors and directors) are cheap lazy incompetent self entitled imbeciles who'll refuse to film another take claiming the useless shit they just filmed can be fixed in post (it can't, it's shit).
Theatres fired all competent projectionists and are now manned by lazy incompetent untrained teenagers who have no idea how to properly operate and set up the sound systems and will more often than not play the films at the wrong volume.
Streaming sites overcompress and overprocess (and overprice) the audio into an even more unintelligible noisy mess.
“Smart” TVs overprocess it even more on top of that, making it even worse. And you're probably using the wrong settings anyway.
In short: the audio is intentionally crap, becomes even crappier in every step between filming and your ears (except, according to people working in sound processing, the sound processing step, but there's only so much you can do to fix crappy digital audio).