They aren't just players. They are way more than that. They are specifically made to organize and display large collections of movies and TV shows. VLC or wmp doesn't have an integration with IMDb to download all the titles, descriptions and cover art for movies based on their Usenet/torrent standard naming conventions.

I'm not saying there aren't plenty of legal uses for Kodi and Plex, I'm just saying that people aren't paying for a Plex pass or spending countless hours on Kodi to catalog their personal home videos.

Neither program is even well suited to that. There isnt even an easy to use interface on either for including custom content.

But drop in a file names the.big.bang.theory.s05e11.720p.brrip.mkv and they both know exactly what to do.