SpaceX did bid fixed-price for HLS, the moon lander they're working on. That project will required developing refueling from an on-orbit propellant depot to succeed.
The thing with SLS is it was supposed to be low-risk, based on Space Shuttle heritage - it doesn't do much that's new. Despite that, it's on a cost plus contract that's ballooned.
Not just heritage, the SLS is basically what you get if you move the Space Shuttle engines onto the propellant tank and remove the orbiter vehicle. Then discard the reusability requirement and you've got SLS.
In the process of this, everything is being modernized. This is where all the cost comes from.
Only if you can do that. Most of the large projects the government does nobody else wants. There is no market for aircraft carriers other than the US government (even if there were not export laws), likewise most space exploration, nobody other than NASA is going to buy your mars rover.
Building roads and private parking lots have enough in common that I might risk some of my own money on a better way [do something with them] them because once I develop it I can recoup any losses selling to private parking lots. Most things the government does they have a monopoly on though, and thus once a contract is done there is no possibility to recoup any losses.
The thing with SLS is it was supposed to be low-risk, based on Space Shuttle heritage - it doesn't do much that's new. Despite that, it's on a cost plus contract that's ballooned.