why is this forced on us and not an option?

[Neuflune]

When you are mounted or using similar objects (such as cannons), you cannot use skills anyway aside from what your mount allows. When you press ESC/stop the interaction or dismount, the hotbar returns to normal.

What exactly is your problem with it? Is it bugged? Does it prohibit you from doing something? Or just the fact that you can't opt out?

Edit:

I suggest disabling the Pet Hotbar if the redundancy bothers you. Under Character Configuration > Hotbar Settings. But if you use a class that has a pet (arcanist/summoner/scholar), it may seem redundant but it's not really a problem IMO. If anything, it makes things easier because Hotbar 1 is usually assigned to Keybinds. Just click the shortcut key on Hotbar 1 instead of the one on the Pet Hotbar.