alt Hacker NewsBased 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
sure, population has grown, but unless density has increased substantially, then on any given ride you're likely to encounter similar number of vehicles than before, not counting major / commute roads of course, but those aren't the ones kids are riding on
also, bike lanes were virtually non-existent back then