This is not a very good analogy because it assumes there is only one option if you build a rocket.
A founder should get funding through selling equity (to VCs) if the risks are high. If the risks are low, the founder get funding through debt, even if you're "building a rocket". Or bootstrap, if possible.
What every VC dreams is to get equity in a low-risk venture. Because the return is high, but the risk is low. That's why there is so much marketing that convinces founders to pay with equity.
A founder should get funding through selling equity (to VCs) if the risks are high. If the risks are low, the founder get funding through debt, even if you're "building a rocket". Or bootstrap, if possible.
What every VC dreams is to get equity in a low-risk venture. Because the return is high, but the risk is low. That's why there is so much marketing that convinces founders to pay with equity.