Nintendo created the tracks... Using their Miis, of course!
Maybe every character has their reason to smash both friends and foes alike with shells harder than rocks, blast them away with thunders and bombs, run them over, and otherwise causing them horrible, but unseenable cartoon wounds.
Maybe Mario, Luigi, Peach, Toad, Daisy *list goes on* and Rosalina, you know, the 'good' characters, just want to play around, while others, like Bowser, Wario and Waluigi want either victory or the gold cups, who knows?
...Or maybe everyone got mushroom poisoning and one of them said 'let's get go-karts and imagine that there are actual tracks everywhere and start racing around while blasting each other into oblivion!' And the rest answered 'Sure, why not'
And so, Mario Kart was born xD