This kills the joke, but:

It's all a stage play. SMB3 was the first (and maybe only) game to come out and say this outright, but it makes too much sense in all the others as well. That's why everyone is still friends and goes out go-karting and playing golf and tennis on the weekends. That's why we've "defeated" Bowser two dozen times including dropping him into literal magma and then he's back again next game.

No one was ever in danger, it's just, soap operas are the main export of the Mushroom Kingdom. They sold us a story about love, danger, and the fury of a righteous man, and we've been eating up the sequels for 30 years. It's Mushroom Kingdom Star Wars.

Bowser being in Mario Kart is like Osama Bin Laden being in NASCAR.

Nintendo made it canon like last week; but Mario and Peach are "just good friends".

Or recreational tennis.

Or the obscure Mario Teaches Typing. Like, why is a plumber qualified to teach typing?

Mario believes in keeping politics strictly out of sports.

It's either you, him, and your girlfriend racing or just him and your girlfriend racing.

just because they break each other's balls all the time doesn't mean they can't be friends

i guess it is funny, but then again what kind of plumbing problems led one into taking part at all ?

I think it’s a turtle

They're really in a poly relationship and Mario has a cuckolding fetish.