I think bikes have always been more or less disposable.

Back in the 70's and 80's, it wasn't uncommon to find a bike at the dump, sometimes in reasonably good condition, but usually low-end department store bikes. Also kid's toys like tricycles.

One of the issues impacting bikes s that they're often kid's toys. And kids grow up quickly, so they can use up a bike every year or two. The bikes often get stored outside, abused, and generally destroyed. People find it more convenient to buy a new department store kid's bike than to buy a used kid's bike that needs new tires and a full overhaul.

That may also teach kids the lesson that bikes are disposable trash (assuming they don't just throw out their teenage bike and replace it with a car).