Put yourself through an invasive medical check up, a police interrogation, potentially a lenghty legal process, likely on "he said/she said" basis, open yourself potentially to a suit or countersuit, all because you regretted having sex with someone? I'm sure it happens, there's 8 billion people and some of them are that mentally unwell. I'm also quite sure the number of such cases, especially going anywhere without the accuser quickly confessing they're making it up, is about as frequent as spotting unicorns
Pretty much every study ever conducted came to the conclusion that actual false rape accusations are incredibly rare