While that all sounds relatively simple, it doesn't address the
contamination of the outer joint. Once a boot has torn or split, water,
dirt and other contaminants get into the joint and will eventually
destroy it, if they haven't already. Additionally, sliding the new boot
along the halfshaft is a guaranteed way to drag dirt and rust into the
joint. Also, by pulling the boot back on the inner joint, you're risking
contaminating it, too, which is a strong possibility if you're working
under the car. If you're going to do the job right, it requires pulling
the halfshaft off the car, completely the entire assembly of dirt and
rust, cleaning both joints of all grease (and any contaminants that may
be in it), installing the new boot(s), greasing the joints, then sealing
them.

In other words, it's much faster and a LOT less hassle to simply pop in
a new halfshaft after pulling the old one out. More importantly, you
KNOW that the joints are free of contamination and should last for tens
of thousands of miles, at least.