I have a 285 lb dart 18 in my backyard that was built in ~1980. It was deffinitely used as a "beach boat" Prindles and other beach cats were made light but durable.
The question as always is cost.

S glass is very expensive today ~ 6 times the cost of E glass in some cases
Epoxy is 3-5 times the cost of cheap polyester resin. vacum bagging and resin infusion has alot of consumables that are not cheap
Good quality carbon masts are expensive to make.
The bigest variable is labour. It takes alot more skill to build a relaible lightweight boat.

Are people willing to pay the extra costs associated with production?

Eric