It kind of is. It could be implemented either by integrating it into the pop-up alerts, so you type your reply and then hitting send closes the alert and leaves you where you left off... Or in the drop-down notification center, you could have an action (say, tap and hold the alert), tap quick reply, and reply your message right from within there.
The point is, having to switch back to the messages app every time can be annoying, depending on what you're doing. Some apps don't like to be interrupted by switching.
OP, I suspect the reason is a combination of Apple's big head, not wanting to introduce convenience features very willingly (especially if they originate on another platform), and their obsession with very linear methods of using devices.