And even among those who do notice, how many people are really bothered?
There's a great anecdote from London in the 1930s. Remember, those were the days when the cinema was an ornate 'Movie Palace' and people actually got all dressed up to go see a film.
(I'm sorry I can't remember the exact details of this, but you'll get the idea.)
One night an eminent ornithologist was at the pictures with some friends. Suddenly, a bird singing in the background on the soundtrack caught his attention. The expert looked perplexed, but nothing more. Then, a short while later the bird-call came again -- and that was that.
'What are they trying to do?!?' said the professor, getting to his feet. 'This is preposterous! That's obviously a _____ singing in the background and this story is set in _____. There is of course absolutely no way that there could be a _____ in _____!' -- and out he went, in a huff.