The problem is that while the MO and the MO rabbinate respect the charedi and charedi rabbinate, it does not go both ways.
There is a halachic difference between covering your hair and covering your elbow. Some rabbis permit not covering your elbow, but others don’t. It’s a halachic issue and people should respect other’s opinions.
As for why many MO might not do certain things, as was pointed out before, many people use MO as a catch-all term, so MO will encompass the ultra-RW branch to the barely religious side of the spectrum.
Hakatan, just out of curiousity, if you live in Ch”ul, do you eat in a sukkah on Shmini Atzeres? I see many supposedly frum Jews openly violating halacha by eating indoors. That’s a lot worse than possibly violating daas yehudis and not covering your knees more than 4.5654″ inches below.