There are a lot of different mobile chargers, if you don't like the specs on the Tesla charger buy a different one. Though do beware that cheap 50A receptacles cannot handle 50 amps continuous. They are for stoves (max 40A), or welders (low duty cycle since you spend more time in setup then welding - assembly lines use better receptacles)

I can confirm this. Our Model 3 doesn't charge as fast using a NEMA 14-50 plug connected via the Tesla-provided mobile charger.

When we moved to a new house, we bought a Tesla wall charger, and it indeed charges at higher amps, but I don't know if the extra speed has necessarily been worth it since we primarily charge the car overnight.