“30 feet is marginal.”
Actually, there is a pretty high risk of death from that height, especially if you are not close to a hospital. True, if you are over 40 feet, mortality starts cracking 50%, but there are other variables, like what kind of surface is the landing. If you are not killed, you are very unlikely to be OK.
It is also a sickeningly scary height for most people to climb. It is a serious obstacle to the great majority of people.
But probably more than that, it is a practical height to build, because they will use precast concrete panels, which must be set into the ground at least six feet (more in places with a slope). If you can’t transport the one million or so panels needed for a sea to sea wall on regular 40 foot tractor trailers, your costs go way up, fewer contractors would be able to work with them, and it will take longer.
My parents house in India was 2 stories high with 12 foot ceilings and a flat top terrace on top. As kids we were not scared to climb down from the top floor, including the 3’ additional railing wall. So 27’ was no big deal. One part of the house had a third floor room, also with a flat roof on top. When we climbed on top of that, it was like 37’ to the ground, and that looked extremely scary. We never dared to climb down from that side using the outside drain pipes. I still get nightmares, of accidentally falling down from the 3rd floor ceiling, which had only 1 foot high railing wall.