There are many reasons why dialogue is seemingly unintelligible:

Directors (like Christopher Nolan) who believe stylistic elements of sound design are more important than the audience understanding what's happening.

Present day actors who don't have the theatricality of their forebears, whether they're almost speaking their own language (Tom Hardy) or they whisper every line, like Sonequa Martin Green in Star Trek Discovery. (Not a movie, I realize, but she is egregious with the whispering. Jeffrey Wright is another serial offender when it comes to constantly whispering his lines.)

Directors demanding that sound designers shovel more and more crap into the audio mix until movie soundtracks become muddled masses of hundreds of audio tracks. This was not a problem in the analog days, because track use was limited.

The lack of any real standard for audio quality and loudness in cinemas.

The lack of standardization for home viewing. Everyone has a different audio setup, and very few people take the time to adjust the settings so they're optimal for their speakers.

Compression in movie streaming. Sound designers interviewed for the story gave Netflix a high grade for enforcing quality on the audio side, and content created by Netflix is mixed specifically for streaming, but many other titles are not, and you're not gonna get the same thing from Amazon, Tubi, etc.

Personally I use subtitles almost all the time now, and it's not unusual to play movies that have been mixed for theaters and not home viewing, which means there's no normalization. Some sounds are loud AF, and then 10 seconds later a dialogue exchange is whispered and you can't hear a damn thing.

I'm glad someone is talking about this and pointing it out.