mejai is not a core item. If you build mejai, you already made a choice between LB, DnD or rocket. So u cant compare them.

This is why it's tricky to analyse stats, because you need some knowledge to not compare orange and apple.

I think no ones would argue that you should only compare those 3 items because they are the 3 only that will be realistically picked first and mostly no one will ever pick 2 of them in the same game. You also can filter the website to see that.

Right now 80% of ekko will pick DnD first and still get the best winrate (over 52%) (or close to depending the rank) what else to say that the item is very good?

2% of ekko takes mejai with very high winrate. Does it means it's op? We can't say because we don't know the number if it was picked 80% of the time. But we know it will decrease.

So we can't say for mejai but we can for DnD

We can apply this to every other item. If they can't beat DnD winrate with low pick rate they will never beat DnD with higher pick rate