Besides all the reasons other commenters have said, it's because mental health is a pseudo-social phenomenon among teens.

Having a mental illness gets them attention, online and in person. I have two teens, and even though both have diagnosed mental illness due to trauma from their other parent, they still seek, discuss, and revel in self-diagnoses.

If a friend claims to have something, they rush to the internet to do "research," and begin exhibiting "symptoms." Same thing is true with other labels.

We have a dearth of parenting, due to needing two incomes to make a household run. Adult attention is scarce, so teens make up for it with wild claims and garnering attention from other teens. The internet makes it easy to model behaviors. So yes, there is an increase in mental illness, but not the kinds, nor for the reasons the internet would have us believe.