Joe,
The thoughts that follow are not meant as a criticism or indictment but merely underscore what this builder's thoughts on the subject are. From the seemingly large number of reports of cracked canopies, you may as well ask why a replacement for Plexiglas is not used. Proseal, properly applied is truly tough stuff and has a long and proven track record in an extremely wide range of military, commercial and general aviation applications. In the production world, workers are given proper training in the correct application techniques and some aircraft programs require workers to be certified to apply proseal. Even then, mistakes can and will be made. That is why inspection departments overseeing such work are an important part of the production environment. The customer requires it. Not so with RV's because WE builders are the customer. You are right to observe that it "obviously doesn't work 100% of the time" and I believe that as far as our RV's are concerned, the problem will always be traced to some degree of builder error. There is just no way to measure quality from one project to the next and consistency across the board is never going to happen. Sometimes, valuable clues as to why some builders are having trouble with leaking tanks can be found in the very questions that are posed. Many people admit to not replacing bad rivets, convinced they will cause more damage trying to replace than just leaving well enough alone. Sorry, but that excuse would never fly in the production world. As far as wet fuel tanks are concerned, material preparation is very important. Have the mating sufaces been truly prepared to accept proseal? Define CLEAN. Some builders never thought to seal under the rivet countersink, others have not adequately filleted the ribs or stiffeners, still others neglected to encapsulate the shop heads and I might add that particular detail has even been overlooked in premade quickbuild tanks. ANY single mistake in the proseal chain can lead to troublesome leaks later. Without a dispassionate inspector looking over the builder's shoulder saying "That work is unacceptable and must be done over." there is no assurance that any given builder's quality control efforts will be little more than a best effort based upon personal opinion and mixed with equal parts good intentions and an attitude of "good enough."