BMX stands for Bicycle Moto X, with X being short for Cross.
When it started, it was with kids/teens/young adults riding bicycles as if they were motocross bikes, jumping, skidding, ASO.
I bet there's thousands of youtube videos of BMX races.
Have a look at one, and you'll see that in-saddle riding isn't a priority feature.
These days there are all kinds of sub-categories, vert, street etc. But in-saddle riding still isn't a big thing.
To get a decent chance of being able to pedal efficiently, the rider's leg should be almost fully extended with the butt on the seat, hips level, and foot on the pedal in its lowest position.
Sure, if the intent is just to pootle around at little more than walking pace, then it's possible to ride just about anything.
But me, I'd rather walk than ride that way. That knee-to-chin feeling, and cranking away with legs at a sharp angle is just too darn uncomfortable.
Not too healthy for your knees either, if you end up trying to do distances that way.
At 5.8' he's what - 3-4" below average? Well in range for bikes that'd be a lot more rideable.
Mistaking BMX bikes for kids bikes is a common thing.
Sure, due to the frame size, they're often used as such, but it's really not that clever.
The handlebars end up too far forward, the cranks are too long, the bottom bracket is too high off the ground. Everything is engineered(and weighs the part!) to be trashed around and still stay together.
It's just our luck that the kids don't know any better, and that the initial thrill of riding makes them put up with the discomfort.
