I agree that pets need a lot of input and are expensive to care for properly - insurance, jabs, flea/worm treatment, decent quality food, litter, bedding, cages/hutches, toys - it's not cheap and too many people don't bother insuring their animals because "it's just a rabbit" or "it's an indoor cat, it won't get injured/sick".
But I think people will always buy from places like Gumtree/Pets4homes and breeders because some rescues have ridiculously stringent conditions on who they'll adopt to. I think rescues do amazing work and they're great places for animals whose owners pass away or move abroad, or for abandoned animals, but I think telling people they can't have a cat/dog until their children are over 8yo, or unless someone is home with it 24/7 makes it really hard for a lot of families to go down the rescue route.
A rescue near me does background checks by speaking to your vet about how you care for your current/past pets and insists on vaccinations/neutering (all fine), but they also insist on at least two home visits (not easy if you work) and they won't let you re-home animals if you have small children, or if you both work full-time.
Lots of full-time working families can provide amazing care for animals - they can use dog-walkers/sitters, and cats (for example) are quite happy to be home on their own so long as they have litter trays/outside access, food and water, and toys. I understand they want to rehome to good owners, but I think a lot of the conditions are a little too much for a lot of people to abide by.