Agreed that walking is great. However, the positive bone-density effects of lifting are greater than those of walking [1].

>Both aerobic and resistance training exercise can provide weight-bearing stimulus to bone, yet research indicates that resistance training may have a more profound site specific effect than aerobic exercise.

[1] https://www.ncbi.nlm.nih.gov/pubmed/9927006

But does it actually prevent falls or make the elderly less likely to break bones?

I'm not sure, but mechanistically speaking that would make sense, wouldn't you think?

People thought that consuming more calcium would do the same thing.

> While there is a small effect of calcium supplementation in the upper limb, the increase in BMD which results is unlikely to result in a clinically significant decrease in fracture risk. The results do not support the use of calcium supplementation in healthy children as a public health intervention.

https://www.cochrane.org/CD005119/MUSKEL_calcium-for-improvi...

By that same logic, should consuming lots and lots of protein yield bigger, stronger muscles?

Or perhaps in both cases, the body must experience stimulus such that the tissues "know" they need to get at the necessary supplies coming in.

Why build muscles, or stronger bones, if the body overall never does anything to need them? It would be wasted motion from their point of view.