With carbon fiber you get a lot of theoretical nice-to-haves, but you can’t iterate quickly with it. You need specialized tooling for manufacturing plus there is a lot of actual R&D that still needs to be done to manufacture with CF at Starship scale.
SpaceX sees all this effort as a diversion from the goal of actually building a new rocket. Compare with steel where you can just hire a bunch of welders and build the damn thing out in the open.
Falcon will never switch to steel. It makes no sense from a performance POV (Merlins are much less powerful than Raptors so your payload capacity hit is too high) and Falcon is a “dead” platform: only minor incremental updates from now on. Any change to it will trigger recertification with NASA, the USAF and a bunch of clients, which is a no-go. Falcon is SpaceX’s workhorse and money making machine, and it will continue on that role until Starship retires it.