Because for better or worse, they're all we've got right now. As a conservative, I would much rather see private enterprise take the lead. But it's just not happening. The private sector doesn't operate aircraft carriers either, but I'm happy my tax dollars build and operate them.
You assert there is no private research of space?! Do you also not see that government expenditure in this area also crowds out private research by forcibly directing limited funds, and by consequence committing a portion of the limited pool of scientists, as the bureaucrats see fit? My question remains, why do you think the bureaucrats know better how to allocate resources than individuals spending their own money, or money voluntarily entrusted to them?
The private sector doesn't operate aircraft carriers either, but I'm happy my tax dollars build and operate them.
Red Herring. An aircraft carrier is purchased with tax dollars to provide for the defense of our nation, from which all benefit, and is, as an exercise of power, an expenditure of funds authorized by the Constitution.