BMX bikes are small and low because if the seat is low its easier to use your shoes or your feet as a brake by pressing or squealing it behind the saddle against the tire. When you are riding a brakeless BMX. If the seat is low, you can use your feet as a brake by squealing it against the ground when your bmx is brakeless. If You don’t, it’s just the cool style of a “brakeless”.
And also BMX Bikes are built for speed, because all the pedaling is done standing up and saddle is made of one material only.