That's how composite materials work. Without going into too much of an engineering answer, you have two materials that when combined, form a structure much stronger than either material could be if used alone. Sure the resin dries and harden, but it's still only essentially a brittle plastic at that point. The fiberglass cloth is what gives the piece strength along the plane axes, depending on the type of cloth used (uni-directional, cross-weave, etc). The resin is merely used to adhere the cloth and join the fibers into one piece that is then able to withstand stresses and strains.
So when we say we're "Fiberglassing", it means we're applying fiberglass cloth WITH resin. You may be confused since it's common practice around here to put a layer of ONLY resin on the OUTSIDE of the pieces first just to stiffen them a little, at least enough to withstand the added weight of the fiberglass that is applied INSIDE.