Safety.

Friends have lost friends cycling on mixed-mode roads. Drivers don't expect to see cyclists, and the normal traffic speeds involved leave little room for the kind of error that even benevolent humans make all the time, to speak nothing of the many hostile drivers.

I would love to cycle. I love bicycling, and am glad it was an option for you! But I don't believe it's viable where I am without significant structural changes.

Cycling on 45mph+ roads (really stroads, or in-town highways) with no separated bike lanes or sidewalk is pretty dangerous. This kind of road is kind of the normal in most American suburban towns or even bigger cities outside the actual downtown area.

In some cities/counties you can't even ride a bike on a road where the speed limit is over 35 (unless it is a county road with implicit limits)

Detroit comes to mind, not sure if the rule is still on the books tho.