I did not get my own horse until I was an adult. Before that all my riding was at a lesson barn where you were not allowed to mount from the ground. You had to use the mounting block or get a leg up. It was explained that mounting from ground pulled on the horses back. I can see how with lesson horses who get ridden multiple times per day how it can really matter.
I am not short and my horse is not tall, but I still use a mounting block. Though I can claw my way up there (and give everyone a good laugh) from the ground, it is just easier for both of us if I find something to use as a block. I have never been on a trail ride that there was not something to make mounting easier.