They don't know the play calls. THey don't understand things like RPOs, hot routes, audibles, etc.
They watch games on TV only and can't even see an entire play anyway. TV broadcasts like to show the offensive line and QB and little else. If the ball is thrown to a receiver, we get a zoomed in look at the receiver ASAP. Where were the other receivers? TV watchers don't know.

I've seen complaints that the ball is thrown short of the first down marker! BAD PLAY CALL!!! of course, they can't see the other 3 receivers spread deeper on the field. Since they can't see them, they assume they're not there.
Of course, they also don't recognize how often a receiver will catch a ball short of a first down marker before running a few additional yards to pick up a first down.