Comment by wiether
Yes, it's about weight.
Basically as long as it pass safety regulations, manufacturers will aim to make the lightest (and cheapest) parts. That's why they use plastic in low-end bikes and carbon/titanium in high-end ones.
You could put motorbike brakes on a bike, and a set of pads would last you a lifetime. But with so much weight there, the bike would feel unbalanced and hard to handle. So you'll need to put weight elsewhere to find balance. And soon enough, you'll have a bike weighting 50kg. Nobody would want to come near this monstrosity. People legitimately want a bike that they can power with their legs and handle easily when they are not riding.
Keep in mind that a single car tire is heavier than a whole road bike. It necessarily results in less atoms, and faster wearing.
First, plastic frames are almost unheard of. There's been a few models here and there, but it's so brittle and it needs so much overdesign to compensate that it doesn't make sense over simple steel. Second, you have it entirely backwards. The lighter materials cost more, not less, because they're more difficult to work with and it's more difficult to get the same strength out of the frame while using less material. As you correctly point out, fewer atoms -> weaker structure. So designers how to work out the geometry to counteract that effect.