The people who keep finding second incomes are not more entrepreneurial than everybody else. They have started listening for one specific thing: the moment somebody says they wish someone would just do this for them, and actually means it.
Once you are listening for that, you hear it several times a week. At school gates, in kitchens, in group chats. Most of them go nowhere. Two or three a year do not.
A surprising number of small businesses began as a favour somebody had already done four times before it occurred to them to charge for it.