Yeah, but that's because 2018 was a strong year for Democrats and John Bel Edwards managed to hang on. Meanwhile, Jim Justice, elected as a Democrat, switched parties before running for reelection in 2020 and Steve Bullock was term-limited. I wonder whether he could have been reelected in 2020 or whether Jay Nixon would have won in 2016?
The streak will most likely be broken in 2022, as I'm sure at least one of Whitmer, Evers or Kelly will be defeated.
I'd add Sisolak to that list as well. Certainly a possibility he could go down too.