A lot of patents have long lists of other patents they rely on. Sometimes the only reason something hasn't become a real thing is because of unresolved patent claims. And in any case, the patent is still useful because they can work on ways to get around the need for using the unresolved patent requirement.

Ie: If my new-widget A relies on patents B and C, but C won't license it to me, it doesn't mean my patent is invalid. It only means I either have to eventually find a way to resolve that licensing issue with the C people, or find a way to replace C with something that doesn't infringe on their patent.

Until then, my new-widget A is protected from a competitor (or even C themselves) selling it before I can find a way to deal with C licensing.