Why would I buy and manage a whole PC for Jellyfin when Plex runs off the NAS without issue?

With everyone saying Jellyfin can run on a Pi, I would think it would be better optimized for low-end hardware.

A Pi isn't a NAS with an old CPU and minimal RAM which is running a bunch of other things as well. I have run Plex on my Synology, and it's fine, but I also have 8gb ram and honestly always have a better experience running Jellyfin on a small PC. Easier that way, and you can run other services. Before the rampocalypse you could find mini-PCs with 32gb ram and 1tb SSD for like $350, and have great transcoding ability when you are on the go or have multiple streams going.