Problem is, ERA doesn't measure only pitching. Defense is a vital influence on ERA. Scorers decisions are a vital influence on ERA. Heck, some parks are more helpful to ERAs and hurtful to others' ERA. And if a pitcher gives up 8 runs after an error is made cause out #3 not to occur, all of that is unearned, even though that pitcher (and the defense) certainly gave up all those runs.