Using language "correctly" is one of humanity's oldest class dividers. [citation needed, source: me speculating] If you personally benefit from dividing people into in- and out-groups (most of the time you do), saying you must speak a certain way is a great way to get people to self-identify on one side of that line. (Excluding cases where grammar helps with communication, that's "I don't understand you" versus "you sound poor".)

You make it hard enough that someone needs years of expensive education or has to be born in the right family that speaks the right way, and now all we can do it try to meet that arbitrary standard. Everyone will struggle, so the act of calling it out is a choice, rather than a fact. If someone lets that mask slip, IMO it's because they're not worried about being accused of occupying the wrong side of the line, rather than any lack of "trying". Trying sort of implies there is a goal to hit.