Because the worship of a deity, and more specifically, dedicating your life to that deity is different than being a someone practices a trade or skill related to the deity's "portfolio". Not every farmer is going to be a Cleric of your campaigns deity of growth and harvest just as every Cleric of that deity may not be a farmer themselves. It's a shaky analogy, but I think you get the point. Putting too much emphasis on "why wouldn't they do this other thing related to their deity instead."
To be fair you mentioned the need for support structures for the worshipers of other deities, but then went on to assume the worshipers of a deity of arcane power would need no such structure, and I disagree with that. Most deities are going to have some kind tenants and places of worship and that will inevitably entail a clergy of some form or another. Within that clergy there's no reason there wouldn't be fully fledged Clerics of the deity, remembering of course there is a difference between a simple priest and someone with actual Cleric levels.
Anyway, I'm not trying to be critical here and I'm posting from my phone so likely making a mess of what I'm trying to get across. I guess in the simplest terms I just disagree with your assertion that a deity of the arcane wouldn't have a support structure that included wielders of their divine power.