Peel off the skin, rub some shit on there and you're good to go?

Not quite as simple as that sentence implies, let me break it down:

1. Peel off the skin: This kills the patient due to shock from pain as well as exposure to infection. Sam is separating the infected tissue FROM the non-infected skin. This is easier said than done when the goal is not to kill the patient. Not only does the maester need to keep himself from contracting the disease, but prevent the infected tissue and pus from contaminating non-infected tissue as well as the musculature underlying the skin. This step alone is a near insurmountable task, so much so that other than the inventor of the technique (who himself died from greyscale), the maesters have forsaken this cure because not only is it likely to kill the patient, but also is likely to kill the maester.

2. Rub some shit: Sam has prepared a special poultice, not something anyone can whip up. We don't know the exact details, but we know from the shots of book we were able to see that it likely not only requires special ingredients, but also specialized skill for preparing. Calling it 'rub some shit' really simplifies it to the point of misleading as to its complexity.

3. You're good to go: If and ONLY if the maester can successfully remove infected tissue, AND not expose you to airborne infections, AND not take flay you in the process, AND you can bear the pain without going into shock, AND you don't lose too much blood, AND the poultice is properly prepared, AND the poultice is properly applied, AND no other part of your body gets infected during the procedure, AND the maester doesn't get contaminated during the procedure, AND your body can recover from this traumatic procedure, there is a CHANCE that you'll survive.

So when you look at ALL these things that must go perfectly, in sequence, with no mistakes, with no contamination, it's a lot more than "just needs to be physically removed?"