Because predators are older and smarter than your average naïve kid. A few years ago I'm making breakfast for my wife (I work from home) and the doorbell rings - it's two town cops asking if I'm the parent of <my kid's name>. Turns out, some local pedophile had sent some dick pics to my 12 year old kid several months prior on Snapchat and they wanted to interview her. This was the first we'd heard of what had happened. We'd long since removed her from social media (unrelated to this event). And this was after spending countless hours repeating ourselves to death about not talking to strangers online. And yes we checked her phone daily...But Snapchat being what it is (disappearing messages), makes it more difficult to audit. She even told this guy what neighborhood we lived in. Since this was his first offense, the guy got 6 months probation and a permanent restraining order against him. Nothing ever came of it, and she's a few years older (and hopefully wiser) ... and the social media restrictions are still on.
In hindsight I wouldn't give my kid a phone until they were > 15 years old and even then it would depend on their maturity level.