I think the cause is a combination of
1. Lousy PPL training (most students/renters don't know the fuel flow at various power settings, but most PPL instructors don't know the figures for various power settings either because there is nowhere to look that up)
2. Reliance on written records done by previous pilots, without a physical check. This is particularly stupid but after the recent highly publicised failed CAA prosecution case it appears to be entirely legal. It is very much common practice in the PPL training/rental scene.
3. Lack of accurate fuel flow measurement, which takes us back to 1. above, by ensuring that nobody knows what the actual flow rate is...
4. There are a lot of Cessnas without a step on the strut, so one has the hassle of carrying a ladder out to the aircraft, in order to physically see the fuel level. I did most of my PPL training in such, and not once did I see anyone do this. This is no excuse, just a big disincentive to do a physical check.