Shen has a higher base AD than Zed.
OMG rito please buff zed.
This is how stupid you people look when whining about X champion having more base health than X champion. If a tank is going to be intentionally building tank, why give it crazy high base stats if it is already going to be building tank? Especially when that tank has a multitude of ways to tank like shields and dodging AAs. That's like giving an ADC a super high base AD because it's an ADC. This applies to essentially all classes.
put simply You don't load a bunch of hp onto an hp stacking champion unless you want a champion with 6k health.
You don't load a bunch of AD onto an AD stacking champion unless you want a champion with 600 AD.