Output of non-inverting op-amp amplifier less than expected
Why does this result in a 15V output voltage?

More than likely the model of the op-amp you chose has built-in output voltage limiting to +/- 15 volts hence, with a gain of 1 + 50/5 (=11) and an input of +3 volts, it cannot produce a realizable output voltage of 33 volts and hits the built-in end-stops of +15 volts.