wow. you keyboard guys sure are hard on your fellow players.
a couple of points. the key of C (on paper) is simple because there are no sharps and flats and is usually the easiest key when teaching isolated music theory, sheet music, OR teaching on keyboard or organ. It is NOT necessarily the easiest key in every instrument, therefore it is not the first key learned, and thus for example a person playing a Bb sax, may view Bb as someone else may view C. keyword: MAY.
also a person doesnt have to learn theory in terms of C (everybody doesnt get confused so easily) and they learn every key on the keyboard as an isolated key. C#/Db is C#/Db; its own key, not "find C, then go up one". so if I had to learn in C# first it is its own thing, i MAY not have to reference C at all. I personally never do (i use the numbers to label my chord progressions). Its kind of like do you think of the number 2 as "2" or as "1+1"? Now later on you learned that "1+1=2" and the whole mathematical way the numbers interacted, but when you learned 1 through 10 you learned it as 1,2,3,4,5,6,7,8,9 and 10. same thing for the notes.
Guitar/bass is not keyboard, keyboard is not guitar/bass. there are literally some things you CANNOT do in every key, not because its harder and you dont wanna learn/practice, but because you actually cannot do, mainly things using/depending on the open strings. Once in another key, the open strings cannot be used in that way (no longer the same notes relative to the key), making some chord voicings, tricks, effects unavailable. thats just the way the instrument is. if thats a huge part of your style, then thats understandable why they prefer particular keys as opposed to others, even though that is kind of limiting.
lets face it, everybody is not going to be able to play every song in every key. not everyone has that kind of time. i had to play keys for p&w one sunday because the organist didnt show none of the rehearsals, and i was up till 1 or 2am sunday morning getting parts of the song down that i didnt quite have right. so we sang the song sunday morning. no we didnt modulate. did i know the song in all 12 keys? nope. did i minister any less effectively? nope. Did anyone even CARE? nope. was i any less of a musician? nope. but i did go home later and work on it in other keys. yes thats a goal, but there's a little thing called progress. ordinary people grow in steps.
gospel has definitely limited itself to a few keys, thats why many musicians are in the box they're in. when i create songs, my main key is Bb, only because thats the key i naturally sing in (or my attempt at singing) so i can have a reference point. but my second song was in E, my third song was in B, i even had a song in Bbminor. The last song i just came up with was in Bb Dorian (how bout that!)
just an observation
