Sure, the chance of you guessing a previously used private key is non-zero, but it is so unimaginably close to zero, that it is effectively zero for all intents and purposes.

There's also a non-zero chance of you correctly guessing the one atom on Earth I am currently thinking of, and this is still billions upon billions of times more likely than you guessing a private key. There's also a bigger chance of you correctly guessing Bill Gates', Warren Buffet's, Jeff Bezos' and Bernard Arnault's credit card numbers (again, by a factor of billions) than guessing a single private key.