Then we've basically been on the moon for centuries now. Just because we can theoretically build something with known physics doesn't mean we have e.g. the mate

Then we've basically been on the moon for centuries now. Just because we can theoretically build something with known physics doesn't mean we have e.g. the material knowledge to actually construct the spaceship. Also, there's this issue of slowing down, and of how to get that spaceship with that much fuel out of the atmosphere, and how to get that spaceship back.... I would say we have a long way to go before claiming we "have" a fusion drive in a spaceship.