Because the fuckers producing the shows make the music and sound effects 5x louder than it needs to be but the dialogue half as loud as it needs to be.
It's because they're mixing dialogue for the center channel speaker. Most people don't have a center channel on their TV or sound bar, but some "Dialogue Mode"s will exclusively play the center channel and drown out the sound effects. It's a trade off, but one that most manufacturers don't even give the option for.
For me, at least, it's the fucking bad audio. So goddamn often the sound makes someone difficult to understand so I watch most things with subtitles.
I don't have to do that with games. Why? Because I get separate volume sliders for music, sound effects, and speech. Trouble understanding just means I need to adjust those to make the speech louder over music and fx.
Why in the hells tv and movie audio tracks don't have this separation I don't understand at all.
There's a couple reasons why I use subtitles all time. Firstly I'm getting older and can't hear as well with background noise. If my wife is banging around in the kitchen I can't hear dialog from the TV. With subtitles on I don't have to mess with the volume.
Another issue is media producers (TV and film) have this idea they need to blast you out of your chair with sound effects and music. So if you turn up the volume enough to hear the dialog clearly, you're going to get blasted by everything else. Trying to manage that with the volume control is damn near impossible. Interestingly I've noticed "dialog boost" appear on occasion in sound track options from my streaming provider. I use it when the option is there. That kind of indicates a global problem.
An issue related to sound leveling is actors used to come out of theater where they learned to annunciate loudly and clearly. It seems actors don't get proper stage training anymore and now it's okay to mumble and fail to annunciate. A decent director should never allow that.
I always use them, otherwise I have to hold the remote the whole time and keep changing the volume. Watching Silo right now and there's so many whispering scenes I'd never be able to make out.
I've started turning on the Subtitles, as it seems as if newer shows and movies are harder to understand. At first, I thought my hearing is going, but not only can I hear the music in the shows just fine, I can watch old shows and movies from over 10 years ago and understand them just fine. In my opinion, it is as if they are putting less volume on the vocal tracks, or maybe using microphones or recording techniques that are not ideal for the spoken language.
It's actually voice technique for the most part. Traditional old microphones weren't as good at recording so actors were specially trained to speak and articulate well to get past the interference and background noise since filtering wasn't really done. As microphones and technological advances improved so did the microphones and the need for special articulation became less and less until somewhere in the 70s or 80s the culture in film shifted to normal voices to have better immersion between the audience and the movie scene. And it just went from there. Nowadays our microphones are so good that even whispered conversations in intimate scenes can be well recorded. So the actors basically just use their everyday voice or try to emulate a real accent which are often slurred.
For the perfect example of the whisper problem watch any episode of Star trek discovery. The main character whispers almost every line and it's very disturbing.