[2023] What's up with the small amount of AP item variety?

The game isn't really balanced around there being access to a % shred MR item. Black cleaver exists because:

1. A staple of basically every comp is an ADC, who's job it is to kill frontline first rather than backline like most AP champions
2. Champions gain more armor per level rather than MR per level. In order to make a % shred item, we would need to increase base MR, which would in turn make situations where you don't have the item weaker.
3. Many AP fighters have % pen in some form built in. Rumble E shred, Gwen true damage, Morde % pen passive, etc. The game is built around % pen not being accessible already.