Cause they are a catch.
It shows success in an endeavor without being braggy, patience because typically it takes a while (and persistence), free time (takes a while), a hobby (can be expensive, and can take a little time dedication to get your gear the way you want it, and also that you have means to do things that are not typical “dinner and a movie” activities.
Fishing is a calm, relaxing past time that is non intimidating, can show that you have some skills other than ordering at bars, and in general is pretty fun.
I dunno, this is just off the top of my head.
I’m really into freediving and spearfishing and pretty much the only pictures I have are ones with fish. I feel super awkward just being in a picture without a “reason” let alone taking a selfie.
Not a guy but I’m a marine biologist and I had a picture on my profile of me holding a fish to represent my true self. I would have considered a fisherman. My now husband was holding a guitar in his (but his grandfather was a fisherman) ?
well if you like fishing, its a good way to show that.