Yes and no. You're absolutely right that NASA is not building the SLS, just like they didn't build the Saturn V or the Space Shuttle.

NASA started out by setting a goal, designing a mission and contracting other companies to build the necessary pieces. This worked AMAZING in the beginning, because industry leaders at the time had the ability to build space craft, but didn't have the money/resources that the government did. So it was very much a team effort where everyone was a winner.

In recent years though, we've really seen that "investment" by the government pay off, as companies like ULA, SpaceX and others have developed their own launch vehicles and space craft operating separate from NASA. (Now yes, it's true that a lot of money was given to these companies one way or another to help with their development, but there is a very clear shift happening as the space industry becomes more and more independent and sustainable)

The issue is now that NASA continues to operate with the old model while industry leaders are capable of doing some work on their own. Meaning that companies like ULA are now having to dedicate resources to developing a vehicle which might directly compete with their own vehicle, Vulcan-ACES.

The argument can be made that, at this current point in time, NASA's money might be better spent funding the development of large projects already underway such as Falcon Heavy/BFR/MCT/Vulcan/ACES etc. They could also spend money into research into things that industry can't afford to do at the moment, as well as funding companies to build probes or other 1-time-use space craft. But instead, NASA is currently spending a fortune developing a vehicle/system which will only be competing with industry for reaching mars.

I'm not saying SLS and Orion should be cancelled. But I believe that SLS will likely be the last project NASA does like this. The future of space flight likely lies with NASA helping to fund the space industry so that they can do science, and the industry can continue to grow into new markets (space internet, asteroid mining, moon mining... whatever you want).

This is a pretty oversimplified view of things, but hopefully it outlines what the issue kind of is