There's a few problems with this comparison. Dogs and cats are prone to communicable diseases and pests as well, but since we know so much more about them, treatments are more advanced. Many serious illnesses can be vaccinated against early in life. Not so with snakes. And unlike reptile breeders, dog owners don't typically house their pets together in large groups or trade them with keepers in other states or countries. When they do cross borders, they need an array of paperwork confirming their health. Because both of these things are common practice in reptile keeping, and not particularly well-regulated, it's very easy for pathogens to be introduced and spread quickly through a collection. You don't know where all of the reptiles in the pet store came from. Some might be from disreputable breeders and some might be WC imports bringing brand new pathogens into circulation.
The idea of letting them build up a natural immunity isn't really realistic because there's no way for their systems to prepare for whatever exotic bacterial infection or virus that might show up with your next new reptile.