Because it's the CLIMAX of the race. If it started over from the beginning, there'd be no time for it to get to the climax of the song. Starting it from the climax is like a variation of Theme Music Power-Up and really juices you up for that final sprint to the finish.
How do races involving bombs and missiles make it past Sunshine Airport's security?
On a related note, how is it safe to fly those planes with all that lightning? In fact, who thought it was safe to host a go-kart race in a busy airport?
In the Marioverse, 9/11 never happened so they never felt they needed the same type of security we do. The lightning probably only effects racers and nothing else (it's clearly not normal lightning, since lightning normally doesn't, y'know...shrink people) so flying planes should be fine. As for who thought it was safe to host a go-kart race in a busy airport...probably the same person who thought it was safe to host a go-kart race in a mall, I guess.
Why is it that Mario Kart: Super Circuit has so little Retro racetrack representation compared to other Mario Kart games relative to its age? Almost every course (15 out of 16) from Mario Kart 64 has been used, as is three-quarters (12 out of 16) of Mario Kart: Double Dash!!, but only two-fifths (8 out of 20) have been used in Mario Kart: Super Circuit. That being said, the amount from Super Mario Kart is only slightly higher (at 9 out of 10), but its themes and music recur so much that there's limited ability to bring back courses from that game. It's as if Super Circuit is being deliberately withheld from the Retro Cups. (These numbers don't include Battle courses, where there seems to be some sort of attempt at equal representation from past games.)