Why aren’t we using _all_ the factories to make as many doses as possible? There should not be “companies” making these to their capacity. This should be a public effort
I would guess that a lot of the factories that are capable of producing these type vaccines also produce a lot of other important drugs and vaccines that we can't just put on the back-shelf without even greater harm.
You first: why aren't you making this in your garage right now?
For the most part the same answer as to why you aren't making it yourself also apply to most factories in the world. Only a few have the equipment, supplies, and experience to make it.