Why are both a voltage regulator and 2 MOSFETs used for a 3-5V to 3.3V circuit?
U2 is a voltage regulator providing a +3.3V supply. Q1 and Q2 are being used as logic level shifters to translate +5V signals to +3.3V levels. They do not behave the same.

If you have another +3.3V supply you can eliminate U2, assuming your +3.3V source provides the same or higher current, same or better switching noise (if any), etc.