To answer this question, we must look at the advantages and disadvantages of using fiberglass in boating, as opposed to other materials such as wood, aluminum, or steel.
Here's a wooden boat for you
When fiberglass was originally introduced as a composite, it had several advantages over wood for boat hull construction. First and foremost, it was cheaper! Boats constructed fully of fiberglass could be produced for a third of the cost of their wooden counterparts.
Of course, a lower production cost wouldn't mean much if no one wanted to buy the finished product.
Fiberglass is much stronger than wood. Thats good. Fiberglass does not need to be caulked to be waterproof when the proper gelcoat is applied. That is also good. Since fiberglass is stronger than wood, the hulls of the boats may be made thinner, and the area inside of the boat larger. This is very good from the perspective of someone looking to buy a boat for recreational purposes.
Any material used in any application must somehow be molded into the form that it will be used in. For some things, the material may simply be poured into a mold. For others, the material must be welded, nailed, and bolted into place.
Why do I bring this up? Fiberglass boat hulls may be one peice and the entire walking surface inside of the boat may be one other peice. Manufacturers may make an entire boat frame out of 2 parts if fiberglass is used, while steel or aluminum or wood would take thousands. This alone makes fiberglass ideal for the production line.
Now, it is important to note that fiberglass will not necessarily be stronger than steel or aluminum when used for this purpose. Fiberglass has more ability to bend than either steel or aluminum, and it will bounce back to its previous shape. However, at higher impact levels, fiberglass may be shattered while steel and aluminum are simply bent and may be hammered back into position.
As a result of these factors, the smaller the boat that is to be produced, the more likely fiberglass will be the best material suited to it.
Individually, a weight of fiberglass is stronger than an equal weight of steel. All the windows in your home, if processed correctly, could be combined to make a variety of fiberglass objects. Many stronger than most metals.