The coaxials don't have the same max range. And ATGs don't have a coaxial. The ambulance needs to be vulnerable (aka die in one shot) to counteract how powerful it is in a forward position in 2v2s and up.
and the OKW Opel ambulance has 240HP and enough armor to bounce a decent amount of small-arms fire because....?
The only one mobile healing and reinforcement platform in game (commisar and OST halftruck don't have much sense compare to it).
Maybe take a closer look at OKW Feuersturm doc, because regardless of what (1v1) streamers say about the doctrine, it's not meme at all in 2v2 and upwards.