Comment by fluoridation

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.