The stiffness to weight can be around the ballpark of magnesium alloys, but with much greater strength.
So if magnesium frames exist (in minority), why don't fiberglass frames exist?
There is a large disparity between the prices of the two types of fibers. Epoxy is cheap, so that's discounted in the price difference. In the manufacture of carbon, you have to spend a lot of energy in the carbonization/graphitization processes, whereas in glass you have just a one-step melt spinning at roughly carbonization temperatures. The graphitization temperature is much higher, but those are never needed for glass.