Probably because Perforce scales to truly massive monolithic repos while git traditionally doesn't (the insanity going on at Microsoft notwithstanding.)

Personally I feel the best method is using lots of small repos, one for each service or library, that get stitched together by the build system. I know some large tech companies have created such systems and I have experience with one of them working very well (they migrated from perforce). But this is a big change from the monolithic repository model and institutional inertia is very real.

(Perforce will eventually start to hit a wall when you get to the point where money won't buy hardware big enough for Perforce to serve your repo fast, but that's a very long way off for most organizations and I believe there are some mitigations for it.)