When did that become the rule? Why, back in my day, 25% was the max amount recommended to spend on housing. Though that was also back when no one would even think of taking out a 72 month car loan. Maybe one of those new 60 month loans, if you just don't have the money, otherwise stick to 36 months.
And like you, I just don't get it. 1/3 on the house, whatever percentage comes out for the $40K car @ 72 months (granted, one doesn't need to buy new), where's this money coming from? We live in Redmond (WA), and I'm at a loss as to how there are so many newer Teslas parked in >$1MM houses. C'mon, there's only so many of those $500K total comp jobs to go around.