It's the main reason I mostly wear jeans. The five pocket configuration's vertical orientation holds onto its contents better than the slanted pockets of slacks and chinos.
I would really like some slacks with vertical deep pockets. Jeans get so boring.
I'm guessing, but I'd imagine as jeans first became popular in the days of horse riding side pockets really weren't a practical option, the rider's motion would contribute to things sliding out of them.
My complaint is that modern clothes designers have forgotten or stopped implementing good pocket design that was fully solved several centuries ago.
"I would really like some slacks with vertical deep pockets."
So would I, and deep pockets do exist but unfortunately not in mass produced pants that are commonly available (one's regular clothing). I have some army BD fatigues/dungarees with really good side pockets. They're not only deep but they're cut in a sort of a shallow 'P' shape. The P-cut at the bottom forces keys etc. to slide down one's side at 90° to the pocket exit when one is sitting. It's thus almost impossible to accidentally lose something out of them.
The questions are why don't manufacturers just copy this excellent design and why don't more people complain. If people stopped buying pants with shallow pockets the problem would be solved overnight.