Statistics, at their best, describe, explain, and illustrate what is being observed. They provide a platform of understanding and can be used to logically set expectations. Earned Run Average does none of those things; at least not in today’s game.
Yes, back at the turn of the century starting pitchers routinely finished games. So a fan buying a ticket to see Walter Johnson in 1910 could say he gave up a little over a run each game and that anybody scoring 2 or more got to him. Most importantly for that era, the gamblers knew where to place their bets.