Tbf your suggested system does have its advantages and disadvantages.
While it would be more consistent and would allow the grind to ultimstely end with a reward rather than hoping, part of the charm of these sort of loot systems is that unknown.
Not knowing when you are going to get a drop drives you to continue doing it, at least thats what I feel when im playing a more traditional mmo.
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It's similar to the way people get addicted to gambling...... [spoiler]but that's none of my business[/spoiler]