Covert cat distribution network, working as intended.
In my opinion cats should never be kept as singles. They need social interaction with another cat, even if they don't like each other. It's not as bad as guinea pigs who will literally die from depression if kept alone, but it's similar.
I wouldn't say "never" because, for me, it's better to shelter them instead of leaving them outside. But yeah, ideally at least two cats. I've only had two cats, both stray, the first one tried to play with our feet when he was the only one in home as he plays with the second now.