Possibly not able to carry enough chemical fuel to lift the fuel into space and return and do a controlled landing without refueling? That seems more plausible.
Fuel is heavy, and most of it is spent on... lifting fuel.
But the way the rocket works is creating a continuous explosion behind it. The propellant explodes, pushing the rocket. It's not just the rocket equation, but the explosion from a central point BEHIND the rocket that pushes it further.
Having a nuclear explosion for example behind the rocket would surely be enough to push it into space... even on Jupiter.
Yes, this is what I was referring to. Though there's probably another percentage that makes most rocket fuel also insufficient for anything more than LEO.
Fuel is heavy, and most of it is spent on... lifting fuel.