We're talking about Monster Hunter here and not some general rule. And as you just said yourself they're unfitting and shit.
I probably played the games five times longer than you ever will in your life and still remember and value the good PS2 online days, whether I'm proud of it or not, so let me tell you about my experience:
Ledges lead to people online camping near ledges to lure the monsters and damage it with cheap jump attacks because they are too bad to fight properly And not just random people, also people from here. It's basically impossible to enjoy this game when your group doesn't consist of nothing but elitist fags and a) I don't even want this and b) it's hard to find such a group.
And of course now with X they don't even need leges anymore, they can just spam retarded jump attacks that are 100% braindead but more than enough to kill a Savage Deviljho with a great sword, even if you're fucking terrible at the game.
People cannot decide where they want to fight and opinions differ. It leads to monsters climbing all the way down so you have to follow it all the way down. Then it gets aggro'd by someone who is still on the "mountain" and you have to follow it back up the mountain.
Ledges lead to monsters phasing through environments making it impossible to even hit specific body parts, either because their head is suddenly too far up or stuck inside a wall or whatever. This is particularly frustrating when actually using a weapon that requires precision and wants you to hit the head (hammer, hunting horn, great sword with punishing draw etc.).
Climbing is not fun. It's a waste of time. It essentially provokes bad players making even more mistakes and the whole group has to pay for it since it's wasting everyone's time and isn't fun for anybody.
Not even talking about mounting per se which is fucking retarded and pointless for its own reasons.
There is a reason basically everyone prefers moderate areas, especially for speedruns. They're just more fun and you can focus on what the games are supposed to be about; the combat and nothing but the combat. Do "curazy" games like Bayonetta put mountains, bumps and holes everywhere? No.
It's not an action adventure and whoever is responsible for this shit should get fired.
I understand that you don't have a problem with the vertical level design if you're mainly playing braindead weapons such as long sword, dual blades, great sword and guns.
This series is heading in a extremely damaging and horrible direction no actual fan should be happy with. There was one thing that always separated it from other action adventure games and that was how important positioning was. Now with retarded shit like this 50 iframes dodging positioning is basically as unimportant as it is in Dark Souls and you just have to press a button the moment the monster attacks. It damages the core gameplay. Forced verticality damages the core gameplay.