What reason is this, besides the fact that wanding one destroys all the others?
If brick can float, how do we know if stuff is connected? It's sort of hard to verbalize, but if floating bricks could be created anywhere, we have a bunch of floating spam everywhere. More importantly, blockland loses it's identity. Despite what some people may think, blockland is grounded in reality, at least structure-wise. When floating bricks can be planted easily, structures stop becoming "structures" and become multiple objects in 3D space.
I told you it was hard to verbalize