Why do some guys flirt with girls but never actually ask them out or ask for her number?

So I’ve been in this situation a few times. I meet a guy at a bar or social event or etc, and we do some (sometimes very heavy) flirting, or even have some physical contact. But then nothing really happens after that. They don’t ask for my number or ask me out. Why?
Why do some guys flirt with girls but never actually ask them out or ask for her number?
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