I wonder if it's because there are many pools, especially older ones, that were plumbed with odd sizes, and the tapered fittings help to accommodate insertion? Just a wild guess unless there's an engineering reason for the shape.
Interesting design. Never saw that before. Perhaps the tapered shape just makes it easier to push into the pipe farther (?) More likely what TS said - more easily accommodates different diameter pipes. Same for tapered corks I suppose.
You are rarely plugging directly into a schedule 40 pipe that has a consistent I.D. You are plugging return fittings glued onto schedule 40 and skimmer holes whose inside diameter can vary. A tapered plug can seal against any manufacturing variances.
These plugs are generally used to pressure test (at 60 PSI) the same type of PVC pipes that are used in pools, so I’d imagine they’re equally capable of deforming to accommodate variances in ID. By the way, my returns are just the pipe with no fittings or threads. My skimmers are threaded, but I use Gizzmos in those.