It is just to keep the first place from getting too far ahead.
And don't give me that "punishes people for being good lol" nonsense. Unless you get hit by like 3 in one lap (which happens occasionally unfortunately) you'd be BAD to let second place catch up just for that.
The s*** that second place has to deal with from behind far outweighs what first has to face from blue shells and what measly items second can get.