Some Carbon frames do use glass fibre as an interface to separate metal inserts from carbon. This avoids electrolytic/galvanic corrosion. No frames use glass fibre in the weave. Glass is heavier, weaker and the cost of a frame is in its layup, build, and the mould. materials make little difference.
Some forks use a layer of kevlar which is weaker, but does not snap.
The big advantage of C over Al is not weight, it is in aerodynamics. You can fabricate far more slippery shapes from C, and for racers, this makes a bigger difference than a couple of gramms in weight.
The big dissadvantage of C is that it is quite soft and vulnerable to surface damage.