My guess is that those stats aren't "need to know" information.
It just adds unnecessary cluttering, in a separate window.
How do you counter lifesteal/spellvamp?
With Grevious Wounds or doing more damage than they can heal.
How do you counter flat armor/magic pen?
By building more of it to offset the flat value.
Your stats are relevant and they are shown because you can micromanage your own builds and damage. However, micromanaging the enemy build is out of your hands and showing you all their stats doesn't help you make a choice. It's just unnecessary information.