system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the belt the symbol of victory in the fighting world? Why no trophies or medals?
gpt
>It is believed that the first championship belt was awarded in 1809 to Tom Cribb after his defeat of Jem Belcher.

Basically, someone did it first, and it just caught on.  As someone said, not all combat tournaments use belts.  But lots do just based on a tradition started (I guess you could say) somewhat randomly.