Many of the weirder-looking lineages (e.g., Tully monsters) from that era went extinct, root and branch, and most of the surviving "weird" animals live in niche

Many of the weirder-looking lineages (e.g., Tully monsters) from that era went extinct, root and branch, and most of the surviving "weird" animals live in niches that don't overlap with humans very much (e.g., extreme ocean depths). I think it's likely that non-weird (to us) body forms are those that are better at surviving in non-weird (again, to us) environments. You see this kind of thing in human activities, too. Most all planes look similar today compared to the bizarre variety of planes that were used in the early days of aviation (you don't see many triplanes around nowadays). There again, the exceptions tend to be planes that operate in some kind of weird modality (e.g., stealth bombers, or the Gossamer X series of pedal-powered aircraft).