Why can't we use old ICBMs for space travel?

With a small enough payload, some of these could place material into low Earth orbit, but with the present payload weights, they are strictly good for sub-orbital 'inter-continental' trips. It would take more effort to retro-fit them for use as space delivery systems that it would take to actually build new, proper launch vehicles.

That's my guess...but hey, I don't know the classified details...and I am not a 'rocket scientist'!