Rockets can be built of out steel (STS SRBs are steel) but not any steel- the average steel you usually get is stuff called "mild steel", and for anything other then car body panels and thick archectural structures it is utterly useless.
You'd probably need maraging steel (used in applications such as fighter aircraft landing gear) which is extremely high-tech; if available at all in a country like the US, it would probably be prohibitatively rare and expensive. You'd be better off by making the fuel tanks out of glass fibre, however, this incurs a mandril and application/curing process.
Rockets can of course be built around thrust and material inefficiencies, although the engineering and manufacturing involved would still be incredibly complex and expensive.
An open-source kit plane sounds like a very good idea to me. A spacecraft is simply too complex and intricate to be taken so lightly.
AFAIK several very simple kit plans exist for homebuilt aircraft, and I could certainly see a market for open-source plans.