Because they're not trout. Fallfish are actually prettier than stocked fish. Had to look up fusiform but your description is quite accurate FS. I think they look cool and have neat fins. When they roll the flash is bright due to their silvery sides. Their scale pattern looks a bit like the scale pattern on a tarpon (obviously head shape and body shape are different) (also size). Fallfish are my favorite out of chubs, daces, and suckers. Although not the greatest fighters they are often willing to take any fly and will do so with a bit of vigor. Fallfish have often saved the day when the trout fishing was getting rough. They pull some acrobatic stunts too when dry fly fishing, sometimes flipping over the fly entirely (missing it). I don't think they have the best eye sight.
There's a junk (polluted) stream by me that has fallfish approaching 18 inches and they seem to be in good numbers and seem to be the predominant fish in the creek. Don't think many creeks have fallfish as the top predator. I enjoy going there in early March and hitting them on massive sucker spawn. I would always try to go out and catch a bigger one. My best is around 17 inches, taken on a hopper. I need to get back to this creek to see how it's fishing, having fished it consistently since 2019.