The fuel sits on the floor of the intake and evaporates into the oncoming air stream.
Fuel doesn't really leak into the cylinders so much. Heck even when running a good bit of fuel is simply sliding around on the intake floor, kinda oscillating in the pulses.
Cranking, the engine isn't producing enough of a signal to the idle circuit of the carburetor to properly pull the fuel mixture, yes it gets some, but a cold engine needs to be pretty rich in order to fire.
Same reason you have a choke plate, the air restriction under it causes a greater signal, thus it pulls fuel more easily.
Due to the design, certain carburetors are easier to start cold than others.
I consider the Autolite carburetors to be pretty much the easiest to start, seems I can leave one for 6 months, crank it up and the dang thing is running.