I'm pretty sure several people have made the double het babies but I don't think anyone has had time yet to make the predicted 1:16 double homozygous albino pied.
There are several reasons why a combination might not work but none seem very likely and there is no particular reason to believe any are at work in this situation, I think there just hasn’t been enough time yet to grow the double het pairs up.
If two genes are close together on the same chromosome then it becomes difficult (requires an odd number of crossovers in the small distance between the two genes) to combine both mutations on a single copy of that chromosome so it's difficult to get more than two copies total of the two mutant genes. In this scenario you could produce double hets but then when breeding them together you would tend to get only albino, pieds, and double hets (no normals, albinos het pieds, pieds het albinos, or albino pieds). There are only so many chromosomes and lots of mutations so we will probably eventually run into this scenario but it's not highly likely for any particular pair of mutations.
It is also possible for the appearance of one mutation to cover the appearance of a second mutation. For example, if you had a leucistic stripe how would you know it was striped short of breeding trials since the leucistic part already takes away the color that would make the stripe? Both genes can be there to make the genetic combination (genotype) but one appearance (phenotype) might mask the other appearance. Since a piebald has dark pigment areas it should be possible to tell if it's also albino.
It's also possible that the combination of two mutations might be lethal. Again, this doesn't seem particularly likely. However, remember that mutations are messing with the normal chemistry of an animal and perhaps the combined effect of two such chemistry changes might be worse for the health of the animal than either change by it's self.
As far as too many ball python breeders ... what gives you that idea? When ball python morph breeders are able to come close to supplying demand the prices will fall to the price of normals which are wild harvested in huge numbers compared to the few captive bred. Certainly they aren't the only snakes where breeders can't come close to meeting demand so pick the species and type you like best but no need to worry about too many ball python breeders any time soon.
