Kitten, I used to be able to mount from the ground. Since I had a very bad riding wreck when I was 30 y/o, I've been unable to do so. I can't get my leg high enough to get my foot in the stirrup from the ground anymore.
If I fall or get off during a ride, I have to find a log or something else in order to get back on. I generally don't need much more than an extra two or three inches, but without that boost I'll be walking back!
Besides, using a mounting block is better for a horse's back. They're not being wrenched to the side while the rider gets on.