There have been 2,664 pitchers who have made at least 30 career starts since 1901.
Three of those pitchers -- or one out of every 888 -- own a career ERA below 2.00. Two of them are Hall of Fame deadball era greats: Ed Walsh (1.82) and Addie Joss (1.89). The third is Pittsburgh Pirates superstar Paul Skenes.
The chances of Skenes, who has made just 39 career starts, remaining in that class are slim. That's nothing against him. It's the reality of math and the era in which he plays. The careers of Joss and Walsh overlapped in the American League from 1904 to 1910, when the aggregate ERA was 2.