To invest, a person needs to find a source of stable income and from his stable income, he needs to extract discretionary income through proper financial management. Because investing with discretionary income is very good because the amount of money left after deducting all your expenses is discretionary income
It is not a guarantee that you must have a discretionary income because you have stable source of income. Some low-income earners have a stable source of income, but they don't have discretionary income because their monthly needs and basic needs is higher than their income.
What a no coiner needs is a discretionary income. Even though, he gets it once in a while, he can invest with it whenever his discretionary income is available. There are some people who are contractors, and they get their contracts once or twice in a year. The profit from their contracts is very big to carter for them throughout till when they get another contract. Such people can deduct their discretionary income from their profits and spread the money over several weeks with DCA method, after keeping aside money for their upkeep.
A gambler can hit a jackpot and decided to invest into bitcoin with his funds despite that he doesn't have a stable income. Therefore, a stable income is not a necessity for anyone to start his bitcoin investment.