And if they were guaranteed or very likely drops, someone would stop doing those world quests afterwards.
A system people could equally call "stupid".
They are a bonus, and if you actively do world quests purely for those, then I would argue it isn't the rewards at fault.
I do world quests for a variety of reasons, not just the paragon rewards.
If you hover over the coin bag icon, at least on mine I can see the number of times I have completed each.