In general, why don't items have seperate stats/effect strengths for ranged and melee?

Balance wise, I don't see a problem with this. In fact it should helps much more with balance.

EG. with rylais it could have been: 400 hp 100 ap unique passive: 40% slow and underneath that it says 'for ranged: -25AP 20% slow instead of 40%'

Or with steraks b4 it's massive nerf : 'for ranged: the shield is 50% effective'

It doesn't even have to be a nerf for ranged. If melee were abusing (Hec and Ekko and use it's increased range passive) then you can nerf the item's on hit damage and add 'for ranged: +25% effect on passive' so that for ranged, it does same amount of damage while dealing less for melee

Balancewise, I find this very beneficial. ranged abusing an item? you can change the ranged penalties/bonuses without harming the melee classes. melee abusing an item meant for ranged? nerf the item, buff ranged effect. That way by balancing the item for one class, you don't harm the item for the other. It also prevents the build path restrictions that 'making an item ranged or melee only' does.

The only downside is probably the information overload that it will bring. IE. numbers may become too confusing for new playes (and old). Apart from that I don't see many downsides to it and I feel the benefits outweigh the downside.