All our aeroplanes have Garmin panel GPS - but few pilots are bothered to use them.
That will be because they have the worst user interface ever invented and are a right b****r to use. You can spend an hour or so re-reading the manual before flight, write down the set of button presses you need to achieve something, and you still can't get it to do anything useful.
And, if you are going to insist on spending minutes and minutes faffing around trying to programme the b****y thing you want to do this with the engine off, don't you, you don't want to be doing it at £2.50/minute. So by the time you've started the engine and remembered you haven't sorted out the GPS yet it's too late, so you don't even bother to turn it on.
Further, having drawn the line on the map and worked out the VORs and NDBs you're going to use, which you have to do anyway in case the GPS fails, you've got more than enough navigation information. Why spend extra eyes-in time on a third navigation system that you don't actually need? Wouldn't your eyes be better occupied looking out?
(If you own your own aircraft of course you have two advantages - one is that you've got a better chance of spending enough time with one particular GPS box to learn how to use it, as compared to a different box fitted to each of the school's fleet, and the other is that you can choose to keep the database up to date, which flying schools tend not always to do. So, unlike in a flying school aircraft, when your own GPS tells you you're going to miss that controlled airspace or danger area there's a fair chance of it telling the truth - in a flying school aircraft without an up to date database you always have to cross-check on the map.)