[cstross]

a) it's kind of hard to hide a launch system where the fueled-up booster stack on the pad weighs two and a half times as much as a Saturn V and the launch is visible with the naked eye from a hundred miles away.

b) There is a reason we don't use Trident D-5 SLBMs (or the Russian equivalent) for precision conventional explosive strikes on targets which otherwise require long range bombers with intricate in-flight refuelling arrangements to hit: something about not scaring the other folks with strategic nukes into thinking we've initiated a first strike and hitting the big red end of the world button.

TLDR: even the big guys don't mess with space-based or sub-orbital weaponry. The risk of a misunderstanding is non-trivial and the perceived costs are far too high.