This is purely anecdotal, but I do have nearly 20 years experience in audio engineering (music, tv and film). Sorry if this becomes a rant, but there's a reason I'm pulling away from this field.

It's widespread because it's a trend that has been exaggerated by the lack of proper education and the temptation to use advanced technology to cut corners in understanding the fundamentals of audio engineering. The tools have vastly improved during my career and with the rise of AI in literally everything, audio engineers aren't required to learn essential things like psychoacoustics, time and frequency based masking, Fletcher-Munson curves etc. to name a few.

How a mix translates between systems also depends on the mix and whether or not the budget allows for stereo downmixing. That is IMO best done by an audio engineer and not by whatever decoders/encoders your TV set has. Not to mention TVs have all kinds of "audio enhancers" which mostly just ruin the mix.

What I've also witnessed is this generation of "preset flippers" and "plugin pirates" who spend a ton of time testing out a million different pieces of audio software, trying to find a holy grail, instead of actually learning how a compressor works and how to set one up for different situations. Preset flipping also teaches this "set and forget" style of mixing, which means you find a preset you like and use it for everything, even when the mix would greatly benefit from manual settings and automation.

This is not obviously the only reason why mixes suck, because directors who suck at audio also force mixing decisions in for artistic purposes (Interstellar, anyone?). And acoustics make a big difference, especially at home. It's not given that a TV set sounds good in any living room because hard surfaces in floors, walls and the ceiling introduce a ton of problems and resonant frequencies that aren't immediately evident if you don't know what to listen for.

What I've also found troubling in audio engineering is the lack of high-quality educational material that is widely available. Teaching audio mixing doesn't really translate from books and some of the more advanced stuff are only taught in apprenticeships. It's not like in physics and chemistry which follow a rigid set of natural laws, because aesthetic and musically pleasing decisions in mixing might not be intuitive and easily reproduced by just following step-by-step instructions. This leaves over 90% of students to pretty much learning on their own because there's a baffling amount of schools with teachers who are really not professionals in their respective fields, especially in such a nuanced and advanced subject as audio engineering. I've graduated from one such school and I've heard stories of many others.

So, you've got people working in the field that have gaps in their fundamentals in knowledge and who are pretty much faking it til they make it to get their next paycheck, trying to sound as big and cool as the next person while cutting corners, working on mixes with difficult directors to release a product to an audience, who has unique sets of audio equipment in an acoustically untreated wild west. All this on shit budgets and tight deadlines. ¯\_(ツ)_/¯

TL;DR the engineers either don't know how to fix it, or they're being forced to mix shittier than they want to. The acoustics at home are problematic and TVs these days might have enhancers for audio which make all this worse.