They can't float through the membrane because the membrane it self is a structure which is firmly rigid in its shape but not in its ability to move.
The phospholipid always has the same shape of hydrophobic tails pointing inwards and hydrophilic heads pointing outwards and because of this the phospholipid stay in place.
And because the membrane is this continuous phospholipid structure they are unable to float around in the membrane, the membrane can move but this is because of the fluid nature of the structure.
