The answer is pretty obvious: those plants aren't new and solar wasn't an option when they were built. There haven't been new coal plants built in the US in some years, and the reasons are purely economic. Wind and solar made up 76% of the generation added last year for that reason[1]. The companies have sunk costs in their existing coal plants, but the economics of solar/wind have become so favorably they've been prematurely retiring them in droves.

[1] https://www.eia.gov/todayinenergy/detail.php?id=42495