Because in most countries, a healthy, controlled exposure is what most teens experience, so that they learn how to handle alcohol responsibly.
In almost all of the USA, it's illegal to drink until you're 21.
Not teaching your children how to responsibly use a smartphone is the current problem.
Denying a child a smartphone, and then expecting them to magically be able to control such an addictive substance is arguably worse.
Be a better parent.