Unfortunately due to the time pressures that get put onto instructors, far too many things get missed out during training. In reality we should do no more than 4 hrs a day flying, with plenty of space built in for proper briefing and de-briefing. This won't happen since FI's are paid by the flight hour so need to cram in as much airborne time as possible.
The only way to fix this is to charge more and have longer lessons or have unpaid non-pro FI's. Only the first suggestion is workable in my opinion.
I only do 3-4 students a day maximum, but I'm paid by an airline, so instructing is now a bit of fun for me rather than my only income. Takes the pressure off considerably and allows me to do the job properly and I will admit to not teaching as thoroughly as I would have liked on occasion because of the time pressure.
That is why things get missed in training, not down to the lack of knowledge of FI's.
The system does need an overhaul and only continued pressure from the PPL and instructor community will help it change.
Whilst touring is what many PPL's do, the vast majority turn up, fly around in circles for an hour, land and go home. Rarely do they go touring any great distance so a comprehensive fuel plan is pretty much a waste of time for them, but since the people who use their a/c as they were designed are invariably far more competant in my eyes, then it's not unreasonable to expect them to be able to complete a decent fuel plan.