What is the number one reason for the need for animal shelters?
IMO, the number one root reason is that most people still think of animals as objects to be owned, used, and discarded when no longer wanted. The population of people like me who see them as living, feeling, sentient creatures that need to be respected and properly cared for, is growing however. But we have to pretty much beat those other people over the head to get them to understand that animals are not like tissue paper you can blow your nose in and throw out the car window.
I feel strongly about this because whenever I’ve had the misfortune to talk to someone in your list above, and try to gently tell them that the dog/cat/whatever is not a disposable object, they get SO offended and defensive.
Your number 2 is also tied to old fashioned religious perspectives which are nearly impossible to defeat. There are people out there - generally uneducated folks - who think that spay/neuter is “unnatural”. They studiously ignore the fact that letting thousands of unwanted animals be born and have to be destroyed is also rather unnatural. Or perhaps they think it is natural, I don’t know. But it’s hard to have a rational argument with irrational people.
Way to see the forest for the trees!