“But a couple making $40k isn’t buying a $300k home, they are buying a $150k home and putting $30k down ($120k loan)”
The couple making $40k aren’t paying any taxes so they aren’t in the discussion.
Yes they are - in fact showed you how much they were ($1,800). If they have a kid, they wouldn’t but a married with no kids would today pay $1,800 in federal income taxes.