I have a couple of old aluminum frames and the bottle cage mounts always show signs of galvanic corrosion due to the steel rivnuts. Why do manufacturers not use aluminum rivnuts?
Steel has a much broader plastic deformation range with a well defined yield point, while for aluminum there is practically no plastic zone and the difference between end of elastic zone and fracture is very little.
This means that an aluminum rivnut would always "bounce back" or reach a state very close to fracture, which is not what one desires for this use case.
I'd suspect the strength of the threads is the main reason.
Aluminium threads are known for stripping easily, and torque maximums must be observed. Steel threads are much more robust.
Personally, if I'm installing a rivnut, I slather the outside with mixed epoxy on the basis this will fill in all the voids and help isolate the dissimilar metals. You can also use assembly lube as an isolator.