In 1990's Dan Goldin, the NASA administrator at the time, was looking for ways to return to the Moon for cheap. One of the least expensive variants which was presented included launching multiple Protons and assembling lunar exeditionary complexes on LEO. The quoted number at that time was $1 billion to put boots on regolith again.

Falcon-9 is about as capable in terms of payload as Proton. And even without inflation the launch price is comparable. So, Falcon-9 is quite capable to get people on the Moon. It isn't really considered because there are other options and some additional requirements.