'cause a fair number of the people reading YA fiction are not YA. They're just plain adults.
And once upon a time, you could get a 60,000-word novel published for adults. The how-to-write book actually cited it as the length to shoot for. 100,000 was ridiculous, then. Nowadays, it's normal for an adult work. And I have actually seen a panel at an SF convention with the title You Must Have Clout, Your Novel is Short. You can't get the shorter lengths published now without clout.
You can get a 60,000-word YA novel published.
All right, you can also get doorstops published, like the latter Harry Potter books. The thing is that they do not push out all the shorter lengths.
Maybe people just want to read shorter lengths.
Could wish so, with my metier being short. But I doubt anyone's going shorter. There's reasons for the length. You can jack up the price. Very little of your cost is actually related to length -- paper, ink, copyeditor time, yes, but your rent doesn't depend on how long the books are, etc. So if the book is longer, you can justify the price increase in the readers' eyes -- except perhaps those that just don't want to read a book that long.