In a way, they are. By which I mean NOT that they literally come with the gun, but that a huge number of guns now come set up to be used primarily -- and in many cases, ONLY -- with optics of some kind. How many ARs now come with rail top receivers instead of the carry handle? Most. How many bolt-action rifles are sold without even a set of iron sights mounted? Most, again.
Handguns are the real hold-out, and that's because of how people use them. Most handgun use -- or intended use anyway (not simply range-plinking) -- largely precludes having an optic mounted. But folks are even experimenting with that. Carry guns with very low-profile red-dot sights low-set into specially milled slides, that don't even have iron sights at all. Whether that really takes off, or is proved to be simply not worth the effort and cost, is hard to predict.
But few, if any, shooters really WANT the manufacturer of their firearm to sell them a scope or red-dot sight, too. Too much variety and possibility there to be stuck with whatever the manufacturer locked themselves into.