It is a mistake when they already have a salary but do not set aside any of it to save, to pay an installment such as a car installment or others such as paying monthly bills is a normal thing, besides that some people who have a salary sometimes only have enough salary to meet their needs so there is no money at all that they save or invest.
Not everyone has to invest because everyone's income is different, some do have a salary that is enough to meet their needs and enough to save it, I myself do not invest but I force myself to save. Maybe someday I will invest when I understand it well.
It depends on your mindset because saving is a habit like anything else, if you can't afford to save a lot, start small, then when your salary grows try to save more. Those who decide not to save anything will one day be out of work and if they don't start receiving a pension by that time, they risk being left without a source of income, and what should they do then? You can't start a business or start investing without start-up capital, and if you don't save, savings won't appear on their own.