Like all gamblers, he knows that sometimes he wins and sometimes he loses. As the film goes on, we see how much of a problem Curtis has. He is very reckless and dejected. It's not inconceivable that he took a chance on the basketball match, not caring so much about the consequences.
Sure, he could have paid up. But he's also a high-risk drifter who will bet on absolutely anything, frequently crashing down with a hard thud.
This scene, juxtaposed with Gerry's scene trying to get in to the poker game, shows this nicely. They both gamble, with poor results.
Of course, despite this reasoning, it's clear many people weren't impressed with the scene. IndieWire, in their review of the film, said:
When Curtis bets on a random basketball game, and refuses to pay up,
he winds up getting beaten and robbed. This scene and others like it
have a random, aimless quality. The clichéd ending doesn't help...
...If Curtis doesn't mind letting Gerry burn all his money in
gambling, why didn't he help pay off the loan sharks? The initial
reason for their journey becomes less and less important to the
character as the film moves along...
...Where are the consequences? When Gerry gets stabbed, he doesn't
even need go to the hospital. It's impossible to sympathize with
seemingly invincible characters.
This was one of many reviews I picked out showing complaints about the films arguably "meaningless" trials and tribulations.
So, whilst there is an arguable reason for Curtis' reasons - an out of control gambler with little concern for the consequences - there is also valid questioning of many of his scenes as simply not making sense in the context of his character and the film.