Source? Because I found plenty of studies on the topic, and there seems to be a fairly universal consensus that goes the exact counter of what you claim. I.e., studies state that weight and resistance training helps with improving bone density significantly, while aerobic exercise (walking/cycling/etc.) doesn’t.
Here are excerpts from one of the papers[0], but you can find many more agreeing with that take:
“Prolonged aerobic training (e.g., swimming, cycling, and walking) is widely beneficial to all body systems, but there are clinical evidences suggesting that none of these activities provide an adequate stimulus to bones”
“Based on the available information, RE [resistance training, e.g., weightlifting/machines/etc], either alone or in combination with other interventions, may be the most optimal strategy to improve the muscle and bone mass in postmenopausal women, middle-aged men, or even the older population.