How useful would your car be if it had no gas cap?
Imagine if they built it with a full tank of fuel, but no way to refill it. Would gas stations exist?
That's the way they build rockets.
Do we want to be a spacefaring species? Build an easily refuelable spaceship.
Make it the upper stage of a Falcon 9. About 13 tons and never intended to land once put in orbit. We can make other sizes for other launch vehicles but a small continuously usable ship with about 180 cubic meters of volume and life support for a dozen (not that it would always carry that many) would have a lot of general use. A refuelable SSTO lander would be sent ahead on a low cost trajectory for any mission requiring this addition. The infrastructure to refuel the lander might take a while to catch up with it but is that any reason not to have it? Would it be a reason not to include a gas cap on your car?
Update: The Falcon is fueled on the ground. How hard would it be to use those same connections to refuel it in zero g?