The problem is with the head weights. Yes, other manufacturers offer sub 50 gram shafts, but they are paired with ultralight heads so that the swing weight is not a mile off. XXIO for example.

Titleist has very limited standard stock options on purpose as they feel like custom fit is a better option for most that buying off the shelf. Yes, that means an up-charge on shafts that they would not stock in the factory but yes, it does give you a lot of options on multiple manufacturers or materials/weights of shafts. If the shaft you are interested in cannot be ordered directly through Titleist, there are a couple of companies like Club Champion that will build to your request.