I'm going to play the Devil's advocate here.
Riot doesn't allow this because new players would probably fall for it. "What, why should I spend months leveling an account and grinding IP when I can just buy one?" League has an incredible learning curve, and sometimes people who have played for years have forgotten this fact. There is a good reason getting to level 30 takes a long-ass time.
There's also the suggestion that Riot doesn't like smurfs on a philosophical level. They want you, the player, to grow attached to your account. They want you to see it as your own investment of time into the game; you've earned that IP, you've bought the champs you like, you've climbed the rank with that, you have friends who identify you by that account, etc. They want you to take that account as high as you can in the ranks. This would all lose meaning if we could easily just fork over some money for a brand-new account.