The reason is that, the underlying causes of depression are relative. The younger you are the less things there are that can get you down. If you're a teenager in high school, the only absolute responsibility you have is school, and obeying your family/parents. Everything else in your life is open to whatever your whims desire. Music, movies, friends, love, sports, partying etc. As you become adult you have to worry about work, bills, family, financial security, health, etc. That's on top of the same things you were worrying about as a kid.
Does that mean young people cannot get depressed? Of course not. Teenage depression is huge, because you're also at a point in your life where you lack the life experience and perspective to see how much "less serious" it is than you're making it out to be. There are teens out there who get suicidal, self-harm and legitimately think their life is over because their first boyfriend dumped them, or a friend stopped talking to them, or something of that nature. In the moment it feels hopeless, but if they could only get past this hump, one day they could look back and see how they dealt with much bigger problems without feeling nearly as hopeless as they got older. You never know how good you have it till you look back. There are some people who are of significant age here who have no friends, and are completely alone. There is also the occasional teen in high school who posts about the same thing. The adult 30 years older realizes how good the high schooler has it, but the high schooler is still in a state of depression. Life generally only gets harder, not easier, and while it's almost condescending to tell someone who is young they have their whole life ahead of them, and they are too young, it's kind of true, if they could only gain the older person's perspective.
When I was 21 I started down a road that lasted 5 years where I legitimately thought I had a gambling problem. I worked in college and saved. I had maybe 5 thousand in my bank account. I slowly gambled it all away over the course of a few months, and as my account balance shrunk, so did my psyche. I entered a constant state of happiness and depression depending on what just happened gambling. But one night, I used an online casino, and deposited, 500 dollars. I was down to my last blackjack bet of $50, when I went on a miraculous run winning $5500 dollars over the course of a few hours. I had one of the biggest highs of life. I had done the one thing every compulsive gambler in his life wished he could do, which is actually get even. The next day I tried to withdraw and they wouldn't let me, as there was a 48 hour minimum withdrawal timer on deposits. You already know what happened. I continued to gamble. Lost $25, lost $50, and in a very short span I lost it all. Worse than that I cash advanced another $1000 and lost that too in a single final bet. I've never had a "serious" suicidal moment where I actually tried something as I just don't have that mentality. Far too scared of the unknown. But in that moment, I felt hopeless, guilty, sick of myself, and I really did wish I was dead.
Looking back I can't believe how ridiculous it was in the moment. Because if I was a different person slightly. If I wasn't too scared to ever try anything, I really might have done something. All over a paltry $6500 which is absolutely nothing over the span of a life.