Yes, I read my own books
The first time a student asked me that question, it took me a while to figure out what, exactly, he was asking me. Eventually I realized he was asking me why I, an adult, would read books written for middle grade readers. That was answered easily enough. Middle grade books are fun! At least the middle grade books I like to read are. They have plot. Things happen in them. The characters have amazing adventures.
And let's face it, a lot of books written for adults aren't anything like that.
A Philip Pullman line I often paraphrase (and I may have originally heard this from fellow Smack-Dabber Tracy Barrett--hi Tracy!) is that children's books are the last bastion of plot. I would argue that adult genre fiction like science fiction, fantasy, and mysteries are up there on the ramparts fighting the forces of plotless literary fiction alongside Sir Kidlit, but the criticism stands for most contemporary adult fiction. Too much of it seems written as an intellectual exercise, and not to entertain.
That's why I write--and read--middle grade fiction. Because I expect a good book to take me someplace I've never been, to do something I've never seen done, to wow me in ways I never knew I could be wowed.
A better question might be, why would anyone write anything else? :-)