If you're interested in natural science, particularly marine biology, you'll know that animals in the ocean evolve into crabs at a rate greater than one might expect.
Carcinisation is a form of convergent evolution in which non-crab crustaceans evolve a crab-like body plan. In the furry fandom, we experience
canization and, to a lesser extent,
felization, in which fursona species drift into possessing canid and feline features, rather than or in addition to the features their species is normally known for. I can prove this through my observations over the past three decades of involvement.
"
First, they came for our knots. Then, they came for our claws. Finally, they came for our fur texture." -
every canid fursona.
I don't need to say much more in support of it; the proof is on sites like e6 and FA.
Dragons with catlike fur, deer with knots, lizards with dog-like claws, birds with red rockets;
they're everywhere.
I'm not coming down in favor of or opposed to this; that's not the point of this post.
My point is that it's interesting.
I think that I know why it happens.
Obviously, the species of nonhuman animals with which humans have frequent, close contact are the ones we have domesticated. Farming is a career that not many furries pursue, thus the animals furries are most familiar with are our pets, and dogs and cats are far more common than any other pets.
It makes sense, then, that they would want their OCs to resemble the cuddly, lovable pets that they enjoy having in their lives to the point of some calling them family.
Everyone wants to cuddle and pet a dog and/or cat. Everyone loves dogs and/or cats.
Furries want their fursonas to be loved. It's a no-brainer to me.