Maximum input voltage in buck converter
The problem is the voltage you are applying to the MOSFET gate is relative to ground, but the MOSFET doesn't care about the voltage between gate and ground. It cares about the voltage between gate and source.

As the MOSFET tries to conduct, the voltage at the source (relative to ground) rises, while the voltage at the gate (relative to ground) remains the same.This decreases the voltage between gate and source preventing the MOSFET from turning on further. You have an source-follower and not a switch.

You need to drive the MOSFET gate voltage with a voltage that is referenced relative to the source-pin voltage. Use a floating gate driver.