You'd be quite right if we had plenty of mRNA vaccines available, but, if I'm not mistaken, we are supply constrained and we'll be for at least a couple more months. Months in which people are dying, people who could be saved by the AZ vaccine, of course at some risk - but which is very much preferable to death, I'd say.
The AZ vaccine still needs to be produced, it isn't sitting in a warehouse somewhere. The amount of time it would take to make AZ vaccine is the same as making the other Pfizer/Moderna vaccines. It's unlikely it would increase supply at all.
AZ has been withdrawn in South Africa because it doesn't work well against the South Africa variant, which is also now known to be experiencing community spread in several parts of the US. So, whether the US (which wasn't alone in not approving it) was right to reject it based on information available in the past, it doesn't seem like it's something we want to try to gear up for now.
According to bloomberg the US has purchased 300M doses each of Moderna, Pfizer and AZ. That doesn't mean they actually have production capacity though.
