It’s also because producers and directors make more use of the “dynamic range” that theatre sounds systems are capable of today. They paid a ton of money for that music and they’re going to make sure you hear it. In surround setups, most of the dialogue is mixed into the center channel, which is behind the screen in theaters. At home, if you don’t have a center channel, you’re either relying on the stereo mix and however they decided to balance the speech on that, or if you play the surround version on a nice processor, it’ll probably let you adjust the center channel.
An audio compressor would also help here on consumer devices, as that would allow a variable degree and ratio of dynamic range flattening. Basically, it can form a band of volume and ensure that quiet sounds and loud sounds both fit into that. Sadly, most consumer gear, although already equipped with all the DSP they need to implement this in software, thinks consumers are absolute morons and/or that this wouldn’t be a selling point, so we get watered-down poorly-implemented dialogue boost features or largely no option at all except for the consumer to “ride the fader” and constantly adjust the volume. https://www.nytimes.com/wirecutter/blog/soundbar-can-help-he... has some info on what’s out there today to address this, but yeah: it’s mostly fixable if consumers had control of the center channel mix + a configurable compressor + configurable EQ. There are ways to do this in consumer-friendly ways for folks who don’t know of care to learn about mastering audio for film, but the current situation is pretty bad and it really makes me wonder why so much mixing targets home theater setups vs built-in crap TV speakers. At very least, streaming companies could provide an alternate audio track [algorithmically] mixed specifically for built-in audio devices that has more clear dialogue.
I personally watch with subtitles because I don’t want to miss a word.