Because I only treat my animals for fleas or worms if I am aware of any infestation or suspect they might be infected.
I do not believe in putting chemicals into my animals for no good reason, and with the spot on treatments the chemical is regularly applied and it is absorbed into the blood stream.
The spray is a knock down which kills the fleas on impact, whereas most of the spot ons have a growth inhibitor, which stops fleas from hatching out and don't actually kill the ones on the animals. I have heard too many horror stories of dogs becoming ill after having been treated with spot on products.
The same with worming, I only worm if I believe them to have worms, i.e. if they have had fleas, or eaten dead animals, or bitches in whelp. I also only use Drontal for adults and Panacur for bitches in whelp and most vets now recommend Milbemax, which can be fatal to some breeds especially Collie type breeds. You can have your dog tested to see if it carries the gene which would make them susceptible to problems with any of the ivermectin wormers. which contain macrocyclic lactones.
I do not believe in routine worming or flea treatment for the sake of it, nor routine vaccination for adult dogs.