Because sometimes you're eating some boring-ass flavorless overcooked chicken breast with no skin and you need something to make it palatable distract you from what appears indistinguishable from tough, stringy cardboard. :)

But really, it's just fun. Enjoying extreme spiciness has nothing to do with enjoying food. Think of it as just a totally separate activity.

And I get that plenty of people don't find tongue-numbing to be fun at all. For all I know, there's a gene for it. But for those of us who do find it fun... it's fun!