MOSFET with low turn on voltages
The classic half-bridge buck-boost design uses a P channel MOSFET as shown: -



This means that the control of the P channel device is between 0 volts and the incoming rail (with care to ensure the gate is not overdriven beyond its maximum rating).

You are expecting an N channel device as a source follower to behave the same and, of course, this means it needs a gate drive voltage higher than the incoming power supply in order to adequately turn it on.

Many implementattions do use an N channel device as the top MOSFET but manufacture a positive rail using a bootstrapping technique.