> given how poorly paid athletes are

To be clear, in the US, when people say pro-athlete, they mostly means NBA, NHL, NFL, MLB. (Yes, tennis counts also, but leave that aside for a moment.)

I Googled for minimum contracts in those four leagues:

NBA: 1.1m USD NFL: 660k USD NHL: 750k USD MLB: 60k USD

MLB is shockingly low, but it might be explained by the farm leagues. (I don't know more details.) I doubt anyone who sees regular play in the MLB is making so little.