A lot of the time it's not absolute volume. If you can't hear certain frequency ranges well, boosting the volume of all ranges is just going to drown out the ones you can't hear well.
Also a problem if you're not a native speaker. I am fluent in English and have been able to watch movies without subtitles for 20 years at least, probably more.
But put me in a noisy bar and my error correction suffers a lot. Suddenly it's hard to follow a conversation that in my native language would be effortless.
The worst situation is listening to a loud video on bad speakers. Doesn't matter how much you crank the volume, people are hard to understand because the brain's error correction isn't good enough. Use a good speaker and turn down the volume, suddenly it's perfect.
This is of course a huge oversimplification, and aiming for it is probably what the industry is doing.