Gunpowder is used in all ammunition. You’re probably thinking of black powder, which was used with percussion caps, flintlocks, and matchlock rifles. But commercial ammunition, artillery shells, and even the big guns on naval vessel still use gunpowder.

But yes, you can still load your own ammunition. You generally have to buy the primer (you could make them, but lead styphnate is really dangerous to fuck around with; mercury (III) fulminate is even worse), but you can re-use the bass casings. You can mold your own bullets if you want to, and there are ways of swaging on a brass or copper jacket, but most people buy the projectiles. Then you buy the gunpowder, and combine them to get the desired chamber pressure and muzzle velocity to get the ballistic characteristics you want.

As for the why - Well, the excuse I tell myself is that, generally speaking, the components individually cost less than the complete ammunition will cost at retail. If you load 9mm ammunition, you might save $.01-.02/bullet, which really only matters at higher shooting volumes, but you have much more control over the exact ballistic characteristics. For rifle ammunition, the savings can be pretty significant. IIRC, cost for 6.5CM ammunition starts at about $1.25/bullet, but I can get brass for about $.20/ea, a primer for $.10, and appropriate powder for about $.15, and a Hornady ELD-M 147gr bullet for about $.50 (or a bit less); that saves me a minimum of $.35 each bullet.

(But really? I enjoy it.)