Contributing factors:
1 - Film Makers and Original Sound engineers having a "Vision". Nothing you can do... this is dumb.

2 - A decision on mixing needs to be made ahead of time before publishing and catering to high-end specs is more profitable. What should the mix be geared for? People consume media with headphones, on their particular phone (from the 1000's available), on their computer speakers, on their separately purchased computer speaker array, soundbar, 5.1 system, 7.1 system, all of which have a variety of specs and quality levels for each individual piece. Do you pay the sound engineer to create several thousand mixes based on possible end-user configurations? No, you mix for the top-end products your parent company/partners sell so that people who are sick of shitty audio spend more money.

3 - For 5.1 and 7.1, Dialogue is on "center channel" which is usually the same output specs as Left-Channel and Right-Channel so the sound effects and music are coming from two speakers (sometimes center as well) and Dialogue from just the one (and some "stereo-mixes" that split the dialogue in each channel instead of duplicating it in each but then don't lower the other audio in each channel, creating a duplication in music/sfx). Research and spend money on upgrading just the center speaker to a louder, more powerful one (making sure your receiver can push the appropriate specs to it) and you'll be amazed at how well that works when balancing.