Well, I was just saying I actually know people who did this - stayed for most of the year or multiple years, especially living cheaply in an RV or truck, or staying with a partner, or some communal type place.
They could get away with it because US border officers would know when you entered the US, but not when you left. The Canadian border officials would know when you came back, but not when you left. The information sharing agreement was only implemented around 2018, and even then, not all border crossings had the technology Then came the pandemic, so it probably stopped then anyway
Medical insurance doesn't know where you've been or for how long...and many young, healthy people let it lapse. People spend years backpacking around the world, they don't care about their medical insurance.
It was 9/11 in 2001 that created the department of Homeland Security and I think that's when they began a crackdown on anyone 'suspicious' crossing the border.
ETA: and obviously, before computers, there was diddly squat anyone could do to keep track of people.