20 years ago the building code in MN required them, or some other form of ventilation. (leaving the bathroom fan on constantly would probably meet the letter of the law, but no contractor I know of was willing to try to get past the inspectors). Different states have different codes, so and I am not up to date on what the latest is anywhere anymore.
A lot of places building codes "require them" at a high level, and then more local building codes strike provisions like this, or provide "alternate paths" to legality. It's disgusting.
Like around here, the building code in one city requires a certain blower door score to pass. But doesn't require blower door testing if you say you followed certain "sufficient" practices. There's no way most of these houses would achieve a sufficient blower door test score, but they all "comply with code".
I'm in Kansas City, and even into the $650k range, houses only VERY rarely are built with them.