I have rescue cats. One was from a feral colony, started out as a couple of cats and there were 19 by the time the rescue got to them. When I lost the boy to CKD, I got another boy as a companion to the girl.
He had been with a family until he reached about 6 months from what we could gather, then he started to show adult male cat behaviours - no longer a cute pretty little kitten but a hormonal teen. So rather than neutering him they dumped him on the streets. Apparently this was quite common on the estate he came from, the rescues do a sweep a couple of times a year to get hold of the ones they can.
Unfortunately he was around 2 by the time he was trapped, so in that time he'd managed to contract FIV (cat aids basically) and has some serious permanent digestive issues - both of these mean his life is likely to be much shortened. He's about 5 now, and I'll be very lucky if he reaches 7-8 years old.
I know the OP has found homes for all the kittens, but that's 4 more kittens stuck in rescues now who now won't be offered the home that hers are going to.
Sadly many people anthropomorphise pets and don't understand that for cats having a litter isn't the same emotional thing it is for people having babies. They don't get the urge to have "children" as such, they have a hormonal drive which forces them to seek out sex and kittens are the result. The number they have in a litter, and the number of litters they can have, is due to the attrition rate of cats before they became domesticated. But they haven't evolved yet to adjust to being loved pets and homed. As a pp said, the longer female cats are left before spaying the higher the chances of them contracting various diseases like pyometra or mammary cancers.
Those of us who have seen the results of indiscriminate breeding tend to be quite passionate on the subject, which is why many have commented on this aspect.
But yes the kittens are very cute!