SLS has been in development for a long time, although it suffers from political winds quite a bit. It started in its current form in 2011, when there was no other system in existence that even was close. Falcon Heavy was a concept then, but it would have only been slightly better than Delta IV Heavy, still nothing compared to SLS. They considered other alternatives, but determined that the best thing was to just create their own heavy launch vehicle, and not do a heavy piecemealed approach that was proposed by some.
It's easy to say with at least 3 heavy-lift, and one super-heavy lift, vehicle in development or recently launched that it seems silly to consider any other idea. The fact remains, only Starship is as capable as SLS is theoretically, and only then if you can get it to refill on orbit, otherwise you are better off using SLS. Starship will probably surpass SLS eventually, but it is still a concept. Remember that in 2011, Falcon Heavy was supposed to launch in 2013, and didn't launch until 2018.
Another thing of some consideration is that Orion can't actually launch on a Falcon Heavy directly to the Moon. It's easy to say that you could do two launches, and with some work, it probably could be done to get the payload to the Moon, but in the current form, it just isn't quite workable.
NASA needs to push something forward, and the political winds have dictated using SLS as the rocket of choice. It does make sense for now, but will make much less sense once Starship is launching regularly.
Bottom line: In theory Falcon Heavy, or New Glenn could be used to fulfill SLS's mission, but they are still not quite as capable as SLS, so a complete redesign of the architecture would be required to use one of these rockets. And unless both of them was available, then NASA needs to have their own system to have some means of redundancy.