cars were just as plentiful in the 70s and 80s as now and yet parents weren't nearly as worried about it as they are now
and kids were much more on bikes then than now -- which is a rare sight unless it's parents with their little kids on a Sunday ride in the park
It's not just how many cars there are, but how big they are, how aggressively they're driven, and how much infrastructure there is for bikes alongside.
Based on the census, cars were NOT just as plentiful. The number of cars per household has risen slightly[1] (although they stop keeping track after 3 cars), but the number of households have doubled[2] between 1970 and 2020.
As for the bikes, it's a vicious cycle compounded by distracted driving via cell phone. Less bikes means less drivers expecting to see a bike, making it more dangerous for bikes, meaning less bikes.
1: https://www.bts.gov/archive/publications/passenger_travel_20... 2: https://fred.stlouisfed.org/series/TTLHH