Ah, well, it's not that hard to understand IMO.
Teenagers will often times have trouble and disagreements with their parents. Everyone surely heard of the phrase "Rebellious Phase" at least once, right?
The dream of having your own independence, not needing to rely on anyone or follow other people's rules when you're at your own home... It's understandable many people would long for this when they're teenagers.
... When they grow up, they realize that, while yes, having your own home is nice, it's actually stupidly expensive and they might need to rely on their parents for quite a while before they live alone for real.
Moreover, they also realize that maintaining a house consumes a lot of time, and that cooking, washing clothes, washing dishes and the like also take a lot of time.
Then they also find out that work can be a lot more taxing than school, even if you're working in a field that you love.
So... It's basically a lack of experience with the adult life. It's easy to look at the good parts of it without realizing the downsides of it as well.
And still... I honestly don't miss being a teenager. I'm happier being an adult than I was being a teenager, even if I have a lot less free time, so... Well, when I look back on the things I thought as a teenager, I realize that a bunch of it was naive, but I also am aware that the upsides that I saw back then were still truthful~