It isn't, though. They're separate shows with different titles, both marketed at roughly the same audience yes, but I wouldn't expect, for instance, an entire episode of Hawkeye to be dedicated to character development for Wanda Maximoff or Sam Wilson.
The Book of Boba Fett is a short series dedicated to a single character's story, and dedicating an entire episode of that limited run-time to exploring an entirely different character remains weird and out-of-place.
If I want to do a full binge of The Mandalorian at some later date after it's got a couple more seasons under its belt, I'm probably not going to sidestep into watching BoBF after season 2 - but that's going to mean skipping past several plot and character developments.