Fiberglass is super cheap and is what most boats are made out of now. Balsa wood core, fiberglass shell. Pennies compared to the cost of a concrete pour. Another issue is that concrete is porous so the rebar inside would rust faster.

In the end, there’s no reason why you can’t. There’s plenty of reason why you shouldn’t. Costs being the biggest. Fiberglass builds are just so much cheaper than anything before it.