Main reason for me is that I paid for a lifetime Plex Pass well over a decade ago and it still fits my needs. I kind of like Jellyfin as well, and briefly glanced at Infuse, but at this point I'm just so used to Plex everything else feels weird.
Well, Plex is a media server 1st and foremost. I've never heard of Infuse before, so I did a search and from what I read it is just an iOS media player that can pull from multiple servers (including Plex.) So, I use Plex to manage all of my media and they have native apps for pretty much everywhere including smart TV's, Andriod, iPhone, Windows, Linux, Mac, gaming systems, and a web player. I am currently looking into FOSS alternatives, but for the past 10 years Plex has been the best for my needs.
I actually have a lifetime license for both. The biggest plus for Plex -- for me -- is that the community is so much larger. If I run into a goofy problem, chances are high that someone else has already encountered it and there's a thread about it on Reddit Lemmy.
On my preferred client, the Apple TV, I've noticed some weird behavior for 4k content from Infuse. On one title in particular I get a crazy tone mapping issue where all the colors go psychedelic, but the title works fine in Plex. That said, Plex's maddening audio delay for 4k content on the Apple TV is the whole reason I paid for Infuse in the first place.