So yeah you can fire a bullet so many different ways, a rusty nail and hammer will do the trick.

Well, there's also the not-small matter of aiming, or firing again if the first shot didn't put a satisfactory hole where you wanted. Or just getting the bullet going fast enough to do damage, without a barrel to channel and contain the blast.

It's not like the whole history of firearms after the cartridge was invented was a waste of time.

You can make guns easy peasy

Ehhh... You can make guns. Metalworking in general is beyond most people, though. And, if you want to rifle a barrel, it's not clear if standard machine shop equipment will be enough (that's a pretty small, tight space for a lathe tool). Easy peasy is a bit of a stretch.

but ammo is much harder to manufacture and easier to regulate

People can and do reload their own ammo. I don't know if anybody DIYs their own smokeless powder, and that would probably be highly illegal, but I've heard of people making their own black powder.