in need of a pee before the tanks run dry.
That's certainly the case for me
Also, I've flown a few vintage aircraft where the oil consumption is more limiting that the fuel - you can carry 5 hours of fuel but only perhaps 3-4 hours oil.
Pilots should always be aware of their remaining endurance and/or range, but most civvie schools air an awful lot on the cautious side. One school I visited offered students a fuel planning sheet with a fuel burn stated of 35 litres/hr for a PA28, when the POH would state something more around 28 for a normal cruise level/leaned, and less if you got higher.
Most training flights don't last much more than an hour, and you would usually have at least 3 hours fuel when you taxi out, so it's not really part of the normal flying training to fly right down to minimum fuel.