because the fuel gauges are crap
It's better if they are snagged for not working at all, then you stop looking at them and wondering. I've only flown two different 152's. One had gauges that indicated full until you had about a third in each tank and the other had one side that didn't work at all (stuck on full) and the other side indicated about half full when full. You still look at them though. In fact you're probably better off just covering them up with bodge tape.