Why cant you convert fluid oz to lb?

There's not a direct conversion unless you know the density of the material. Fluid ounces measure volume and pounds measure weight.

Water has a density of roughly 1 weight ounce per fluid ounce, so for it you're reasonably safe saying that 16 fluid ounces of water weigh a pound. But if you have something like Mercury that's about 13.5 times denser than water, 16 fluid ounces of mercury would weigh around 13½ lbs