I mean, this is just because most people have 1-2 income sources and way more expense sinks, so this is just rhetoric.
It is not that difficult, though -- the greatest problem is to develop the habit. You need to make a note whenever you pay for something (there are smartphone apps for that), and have some simple categories defined (that you can refactor later). At the end of the month put your notes together with your online banking reports in a spreadsheeet... and you have the entire picture, which you can later compare to other months.
People who did this usually found some surprises, and were able to save some money. But it takes some time, and if you are employed and have kids at home, time is scarce; it is better to develop the habit sooner.