Why do I get constant voltage across load of single phase inverter even with increasing input voltage?
The information you give is conflicting.  You show IGBTs, but then say you are using N channel MOSFETs.

However, the reason seems to be that you are using NPN or N channel devices for the high side switch, but not driving their gates/bases high.  NPN or N channel devices will always have their emitter/source below the base/gate when on.

Either you need to use a special high side driver intended for N channel devices on the high side, or P-type high side switches.  The inputs of the P-type devices will still need high voltage, but can be easier to drive.  At least you won't need a voltage above the positive power rail to drive them.