You're right, it can't.
So, snap the cylinder home to your heart's content.
I suggest doing it dozens of times a day...
In a few weeks... OOPS!
Ok, smart answer aside, as Sam notes, firing the gun puts different kinds of force on it, and most importantly, in different locations.
When you fire a revolver:
1. The firing force moves out in all directions at the same time. When you snap the cylinder shut, all of the force is moving in one direction.
2. On firing, the cylinder is held front and rear by the crane assembly. The cylinder bolt and crane pivot take very little force, actually. When you snap the cylinder shut the cylinder bolt is slammed into the side of the cylinder bolt window, which can cause both battering of the cylinder bolt AND the window. Also, when snapping the cylinder shut, the entire cylinder assmebly is pivoting at one point, at the front of the cylinder, which means that there's a considerable amount of non-linear torquing force on the crane.
Finally, and MOST importantly, is that not all parts of the revolver need to be as strong as others.
Because the cylinder yoke, pivot pin, and crane assembly really don't take much of the force of firing at all, they don't need to be heat treated to withstand the kind of force that the cylinder does.