They're gearing up equally as fast as you are - One item a week.
They just give themselves more chances at loot they need.
That's literally the most sensible way they can make this system. It's unfortunate that you are experiencing a stroke of bad luck in your 2-3 chances a week, but 2-3 more chances a week doesn't guarantee you an item any more than your original 2-3 chances. And since you still only end up with ONE item, it's literally no different than rolling an item off a boss loot table and picking which of the rewards you get.
The lowered loot drops in Mythic+ just means you have a lower ceiling. The chest still gives the good stuff, as it did in BfA giving several item levels higher than the max drop in Mythic+ dungeons. And the chest is balanced by being 1 item earned a week, no matter how many chances to roll an item you need that you get.
Yes, playing more SHOULD net you more chances at something you need. I don't know why that's hard to understand.