Because power cords and very heavy tools on a metal roof are great...

A decent hammer drill will slice through concrete, and offer far less torque-twist to deal with too.
I have literally never had a situation where my batteries limited my ability to work. Modern batteries will generally charge far faster than I can wear a different one out, even when not using a rapid charger.
Aside from the improved safety from no cords and reduced weight, the "modes" and intelligence built into modern cordless tools are a revelation. You do you, but you're essentially arguing that fax machines are better than email. Have fun in that world, just don't try to sell it to people who know better...