Yes, I read YA books. My book club reads YA exclusively.
I can only speak for myself as to why the YA genre is popular with adults.
I've always loved younger books, especially fantasy, such as the Chronicles of Narnia, Abarat, The Hobbit, Harry Potter, etc. But I recently "discovered" YA books. Back when I was a teenager, the only YA book I recall reading was the Gossip Girl series, which was fun, but when I got to be 17 and 18, I wasn't looking for that kind of book. Teenage girl drama was not my thing.
I read literary fiction that sometimes involved slightly younger characters, but I kind of avoided YA books thinking they would be too immature for me.
I think the first real YA book series I read on purpose was the Hunger Games series and I was so surprised. I picked that book at random for my brand new book club and absolutely fell in love with YA fiction. (I'm not going to count Twilight,
because I merely stumbled upon those books by accident and had no idea
who they were marketed for.)
Now that I've read quite a bit of YA fiction, I can definitely say exactly why it appeals to me. It's so much deeper than adult fiction.
I've read a lot of popular adult authors like Nicholas Sparks, James Patterson, Sandra Brown, Nora Roberts, Christine Feehan... etc. And while I enjoyed them at the time, as soon as you've read one, you've kind of read them all. Each one of those authors has distinct plots and after reading 2 or 3, you are not surprised anymore. With the exception of fantasy, which seems to be less predictable to me, adult fiction is kind of boring to me.
With YA fiction, even the same author can throw so many twists and turns in multiple novels. The books can touch on a variety of things. Using Hunger Games as an example, there's dystopian aspects, a small love triangle, an adventure, conflict, internal turmoil, and coming of age. I can't think of many adult books that would have so many angles (again, with the exception of fantasy and perhaps classics). I feel like YA fiction isn't tied down to genres like adult fiction. A YA book can be a lot of things and the only thing the author has to "worry" about is marketing it to a younger audience. An adult author has to worry about marketing to adults that read specific books, which limits the variety of things you can encounter while reading. And the very fact that YA books are marketed to a younger audience is the great part. All the deep and profound themes are not spelled out. They are hidden among the text so that days after finishing the book, it stays with you and you can't wait to discuss them amongst friends. It's rare that an adult book does that to me, unless it's a classic (and we all know those are discussed at length in schools everywhere).
I hope this trend of amazing YA books stays with us for awhile, because I find it to be extremely refreshing. I love the new ideas presented and new plots involving some of my favorite themes, especially Dystopian themes. It's like rereading some of my favorite books with fresh faces.
Also, I think YA fiction appeals to adults for one of the reasons I mentioned above. With so many adult authors having mirrored plots in all of their books, I think adults are looking for something new, something they haven't read before. And even if adult authors release new novels, it's the same story in disguise, under new names. YA is a breath of fresh air for those of us who have read a lot of books and crave something new.