Tanking meta confusion

I am confused why riot is so happy with the current meta and the current way tank items give so many arbitrary stats for no reason?

like why do tank items have CDR when they aren't necessary for the items? 10% 10% builds into commonly used items on bruisers and tanks which doesn't need cdr to be strong which for reasons I don't understand then gives 20% not to mention about 1/2 of tanks build which also gives 10% cdr

I don't understand why cool down reduction is so easy to come by as a tank when it isn't something that needs to be part of these build paths, they need the armor and the magic resist if they want cool down reduction they should have to buy items with the purpose in mid they shouldn't be given such an easy path to 30% cool down reduction that 9 out of tanks follow and have by the 20 minute mark of a game when they should be getting the items for the armor and magic resist can someone explain why riot is so happy with secondary stats on items that don't have justification having them other then them slapping it onto the item?