I have always had my cats neutered as soon as they are old enough, as it is hard enough to find homes for one litter of six kittens, and must be a nightmare to find homes of more than one litter.
Most people who keep a cat, or more than one as pets probably feel the same and are willing to pay vet's bills for neutering and vaccinations.
Those who keep working cats (on farms, stables, wood yards etc) where the cats are primarily kept to keep down mice and rats frequently do not have their cats sterilised for the very good reason that the cats are semi-wild. Anyone who has ever tried to catch a cat that did not want to be caught knows how difficult that task is. In this sort of mileau either half-grown cats are chased off by their elders - cats seem to have a pretty accurate idea of how much food there is either being doled out or scampering around on four paws, and take steps to ensure that surplus youngsters are chased off. These cats either find some human to take them in, or live short lives as feral cats.
The supply of kittens needing homes is practically nil in cities, but in country districts, due to the cost of neutering and the fact that relatively few people now are prepared to put down unwanted kittens themselves, but probably would not consider paying a vet to do so.
IMO responsible pet owners do ensure that their animals do not have unwanted litters, but not everyone agrees with this point of view.