Why do some men buy me a drink at a bar and don't say much after?

They are waiting for you to approach them and say thanks. Then they'll chat you up

In a time where men and women are equal and paying for a drink and meal should be 50 - 50 , the act of a guy paying is like an extremely classic romantic gesture, like the end of the line gesture, like i can't do anymore to make her notice I'm into her...and so he waits for your move pushing forward.
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The above is just an assumption or a hypothesis, i could be totally wrong, but just keep the idea in your head as there could be a time when other signs and moves by the guy that elevate this hypothesis into a theory.

Maybe waiting on you to make the next gesture? saying thank you, an appreciative nod...something like that. Not implying you owe them anything at all just answering

Maybe they want to get you drunk and are observing you carefully from afar... 😉

Waiting on the roofie to kick in

Could be staying discreet cuz they got a girl already

Intimidated. You're a very pretty woman.