Overdriving LEDs with high currents
Specifications for IRF540N show Rdson = 44 mOhm  alone, and only at Vgs=10V control signal. So you already should see only 30% of what you expected. 

If DRIVER_1 has only 5 V,  the FET can barely commute 10 A of current at Vgs=5V, see specifications, Fig.1 and Fig.2, it looks more like a 100 mOhms with this control level.

To conduct 75 A of current, your wires must be short and have a solid AWG rating. If not, every wire segment will add few mOhms into the circuit.

If, God forbid, the whole test was done on a breadboard, then every junction would add 10-15 mOhms into the loop, and I counted about 7 junctions. This is another 70 mOhms. 

More, the forward voltage of a LED will be quite higher at high 10X- 100X current pulse than the listed "nominal" 3.5 V, so the voltage drop across the transistor net would be quite less than 1.5 V. 

Now you add all this together, and you will have the result.