They let monk do this because they weren't able to add its real functionality. Slow fall doesnt work often, they can't climb walls for free, and step of the wind doesn't multiply jump distance. You can also jump without an action in 5e. And BG pathing doesnt include squeezing, or objects moving, etc. Jump is needed to make traversing the map not fail.
Also, they likely felt that base movement didnt work well enough on their 3d maps. (most of 5e is designed with flat maps), so essentially you can dash to double your ground speed, or jump to increase your ability to move in 3d.
basically, through jump, the base speed is 16 meters for high str characters. monk in 5e has 2x the speed of regular characters, and step of the wind doubles that, so 66 meters is on the table.
I have no problem with monk's having the best mobility in this game.
