The reason I think it's a good standard average is because most landlords or apartment managers require you to make 3 times the monthly rent, and likewise, most mortgages will only let you borrow a certain amount depending on your income. For example, FHA makes sure that your monthly mortgage, taxes, etc., are 30% or below your gross pay, with an exception that allows for 35% if you do not have a lot of other debt. Plus, of course, it's what financial experts advise, so that way you have enough money leftover for all other living expenses and savings.
Then you have other aspects that complicate this "rule" b/c like it's been said, if you have a high income, you have a lot more money leftover even after paying whatever percentage to housing. And people in HCOL areas often have to rent indefinitely, or have a huge downpayment, or, pay a higher percent for housing. Some people are more comfortable basing the percentage on one income, even if they are a dual-income family - which is wise in case of job loss or SAHP'ing in the future.
It's obviously not a strict rule all-around, but I do think most families fall somewhere around that percentage, with outliers on both ends.