As others have noted, this isn't unique to the current times. And as someone who only recently finished being a teenager, I think the issue is primarily social media, especially for girls. The reason is because social media is communication and socialization. They are vital things for almost all teenagers because human beings are social creatures, and at this stage of life particularly, independence is developing, you are learning to become more of an individual and so on. In previous times you might get together after school at a park or whatever because you didn't have a phone, but these days everything is done on social media.

You are judged before you meet someone by your social media page. You can't know everything about a person before meeting them, or really without being a good friend of theirs, so social media becomes a proxy for this, and not a particularly good one because you put what you want on it and thus teenagers try to game it in various ways (for example only posting holiday pics to show how glamorous your life is).

It becomes a race towards the bottom in so many ways and in particular for girls, because so much value is placed on physical appearance (either by others or by the girls themselves) and how 'cool' are you and thus it makes you constantly compare and judge yourself with others. It is no surprise that teenage mental health is plummeting when a lot of time is spent on social media, it constitutes a large part of the status game, and there is no reasonable alternative. This is why I think it's a sole factor because I can imagine you would have the same problem even if you took away other economic/political/environmental problems.

Not to say those also don't have an effect, because they obviously do, but unlike those other things, every teenager wants friends, everyone wants a social life, it's not something only half of them know and the other half don't know much about because they read it on the news or learnt about it from school, it's something that affects every single one of them every single day. You cannot escape from it.

Surprisingly despite being a guy I don't know many guys with mental health issues, maybe they don't want to talk about it to other guys, maybe I am not close enough with them, but most girls I know do have mental health issues, and every single one of them (n~=50), literally without exception, has self esteem and body image issues. And it worries me, a lot, because how can we have it that 50% of the population suffers like this every day for years on end and nothing changes? What will the end result be?