Not all cats that turn up on your doorstep have been abandoned.
Some will have been frightened by, say, fireworks, run outside their normal territory and have been unable to find their way home.
Others will have inadvertently hitched lifts on vans and lorries delivering to their street (by hopping in the back while the doors were open and being trapped inside when the van pulled away without the driver noticing them) and have eventually found themselves many miles from home.
Some will have simply left home because they got fed up with living with other cats. (I suspect that's why my mother cat left home last July. She loved her kittens when they were small but she wasn't very happy living in a house with 5 cats when they grew up, so she went to find someone else to look after her).
Others might simply have been left with nobody to care for them when their tame human died. (I live on my own and, if I dropped dead, it might be months before anyone noticed. My cats would simply go and look for new homes).