The ones wearing jerseys are a mix. Some have sponsors. Some have them for "fashion". Some for practical reasons (they breath very well).
I have one that is personalized to represent me. I have zero sponsors. I also like that they breath very well.
Some in IDPA and some of the other sports see jerseys as a "gamer thing".
That's my take on it from what I've seen and heard.
I've been to matches (not IDPA) where if you wear a jersey and/or shoot USPSA, Steel Challenge, etc. You are looked down upon as a "f#&%ing gamer" and they do let you know you are not welcomed as "one of their crowd". I, frankly, don't care. It fits me.
Interesting that people would look down on you. Maybe I'm just ignorant but all shooting matches are "games". The breathability of the garment makes a heck of a lot of sense. I have this Reebok shirt that was probably designed for people playing basketball, but it is comfortable and it works.
The local matches I go to don't have anyone running around with the jerseys. But I was thinking about going to the 2-gun rimfire nationals, and wanted to blend in. Seems most people have jerseys there. Really appreciate your input.