For a lot of people it is, but for a lot of people it's not. Unforeseeable illness/injury or being a victim of a crime can change someone's financial situation much faster than they can change their living expenses.

If you have a mortgage you can easily afford, but then get decimated by an illness in your family, or an uninsured drunk driver destroying your cheap used car that you need to get to work, and injuring you so that you can't work at your job for several weeks or months, or somebody breaks into your garage and steals all your tools that you need to do your self-employed work, your mortgage doesn't evaporate. You aren't just magically transported to a rented single-wide trailer which is what you can now afford, and your house instantly sold for as much or more than you owe on it before your next mortgage payment is due.

Once somebody gets behind, especially with children to support, getting out of the paycheck to paycheck cycle can be very difficult, and impossible if there are additional rounds of bad things happening to you. Like your business income evaporates because the government orders it closed because of a pandemic.