Because smurfs are allowed. That said, in 95% of the cases, smurfing is pointless. Why play against a few teams where you can stomp? That isn't fun, and it likely just makes you play worse yourself. I have a smurf myself. It's lvl 20 but is only slightly below my normal MMR already.
The issue I see is when people throw games with their smurfs or have multiple people play on them, which is already grounds for banning. Then you end up with, for example, a bronze or silver player that is actually gold or plat in some matches. Then they use that that carry friends in ranked. That kind of stuff should be banned (for both the offender and the person being carried by somebody who they knew would then be doing something prohibited).
Regular smurfs aren't really a problem, tbh. It's just the ones that artificially lower their MMR that are an issue, and rito doesn't seem to want to acknowledge or address that problem anyway.