Why would they build land based missiles? My understanding is that that part of the US triad is basically pork due to the capabilities of sea and air and the fact that they are static installations that can be easily targeted.

I can only speculate that it could make sense for China if their sea and air power are substantially less capable.

It's pretty amusing for US officials to express concern over an increasing arsenal when they are themselves spending over a trillion dollars on that exact thing while they deliberately provoke China over the Taiwan issue, placing US soldiers in Taiwan, which is considered Chinese territory.

https://www.wsj.com/articles/u-s-troops-have-been-deployed-i...

It isn't really a Triad these days. It's more like a hexaiad when you consider the various forms that nuclear weapons can take.

But as to your question, silos benefit from very reliable coms. It's possible to communicate to subs, but it is slow and unreliable. Airpower suffers from jamming and signs of strike, since the planes need to be put in the air and that is observable. There is also a limit to the size of the missiles and therefore immediate range. If you want to strike anywhere on command with little notice, silos are best. If you want to hit foreign target with absolutely minimal response time, submarines usually win out, since most major capitals are by the ocean. But even beyond this consideration, it is good to have a diversity of options to reduce the risk of a catastrophic vulnerability existing. For example, some hypothetical weapon that could eliminate all your submarines without warning.

Good answer, thank you!