First every mage build Rylies, then morello, then ludens.....

Before Rylai's it was . A lot of them were rushing in Season 6 too.

It's not a diversity problem it's the power of the items. As long as mages can all buy the same things and be effective they will just buy the strongest one because there is no reason to get more utility for less damage when you have enough in your kit. It's the same reason why we see assassins with the same items all the time no reason to buy weaker damage items when damage is all you care about. You could make 200 items for mages you will still see the same things on each one because each mage will have their own optimized build that is just the best.

I personally use liandry's first

I am offended, I build Rod of Ages, then Ludens, then Morello, though to be fair I am playing a tanky/mage Swain.

"Diversity" has been dead for years. Every bruiser builds tri or tiamat first. Every ADC built IE first and now Stormrazor first. Every support finishes their remnant first.