The US did look into mobile ICBMs at least twice (the Midgetman system and the Peacekeeper Rail Garrison). Both were in active development in the late 1980s, but were cancelled at the end of the Cold War.

In the end, all of these systems were expensive, politically tricky (Americans don't like to see nuclear weapons being driven around the countryside), and, ultimately, of questionable utility. You need huge areas of land to drive them around on, and while the US is quite large, most of that land is, you know, used (or not good for driving around gigantic missile trucks on). And the idea that a foreign enemy that has the sophistication, arsenal, and accuracy to attack fixed silos would not be able to track and detect mobile launchers seems fallacious in a day of copious satellite imagery. So they aren't that stealthy, they aren't as survivable as a fixed bunker, and they're way more expensive.