Because of all the 'um actually' corrections from people whenever they'd say "Tom and me bought drinks." And not just to the point one starts thinking it's always "Tom and I" - I've had people 'correct' my 'to Tom and me', as well, because they think "Tom and me" is always incorrect.

This is also why I don't make a big deal about correcting others' grammar; it's often a tool people use to feel smarter (and thus superior) to other people. Language is a communication tool; if I know what you mean and there's limited ambiguity then I don't much care if you said 'would of' instead of 'would've'; and certainly not enough to interrupt a conversation to correct it.

Besides, between autocorrect, typos, and the brain's weird word-association tricks, a linguistics professor is capable of making significant grammar mistakes and not even notice, even if they'd know they were wrong if pointed out. So swooping in to tell them "hey you did this thing slightly wrong" in lieu of engaging with their intended point is not meaningful contribution.