It just isn't feasible. Making a vaccine requires specialized equipment. Saying "just ramp up production now" is the same as telling Boeing to just make more 777s. It takes months to build these facilities. Most of the manufacturers are private companies. Who is going to provide the financing? When everyone gets vaccinated in a couple of years, what are these companies going to do with these facilitates once there is no more demand?

And doing productions in parallel with trials is also very risky. Again, it is like starting a production line up while and aircraft is still being designed. A large percentage of vaccines fail during the initial testing. I'm not saying they won't/shouldn't start production while still testing, but who is going to pay the bill when they find out 2 months into production that the vaccine failed human trials or what if COVID-19 mutates?