It's not built into the ROM. iMessage got a huge update with iOS 10. If it was built into the ROM, that wouldn't be possible, because the old version would be, well, read-only.
Either way, iMessage comes by default, and every iPhone is guaranteed to have it. Using iMessage instead of say, Whatsapp means I don't have to download yet another app and sign up for yet another service and search for yet another user(not to mention possibly giving away personal information to Facebook) when I can already use what's already on my phone.
Thanks, Captain Pedantic. Pretty sure folks get the idea despite the egregious inaccuracies in what I wrote. (And, as an aside, I knew that when I wrote it.)