Because unless I have access to the AdSense inner-working algorithms, it is humanly impossible for me to know what Ad/Ad-Network is under-performing to who/what/when/how.
Actually, it's not. You do have the ability to see how much you've earned from each network, going back to some finite point in time.
I did that, saw that the combined earnings for ALL those non-AdWords networks were LESS THAN TEN DOLLARS OVER MILLIONS AND MILLIONS OF AD IMPRESSIONS and blocked every single network but Google's, plus blocked any FUTURE networks.
I for one didn't need to have access to an algorithm to make that call.