One of the most expensive components of your computer is your monitor, but it is also one of the most standardized and longest-lasting. Think about it: pretty much any monitor bought in the last 20+ years* (including CRTs) can be used with any computer or video card on the market, requiring at most a super cheap adapter.
But with an AIO, you lose this major advatage desktop systems have over laptops. You're still paying for the monitor, but have little to no choice in which model it will be, and you will only ever use it with this one computer. And when the computer dies, the monitor is finished too (even if it's in working condition).
* Yeah, I know analog/VGA is starting to go away. I still holds for digital.