Return on capital and deprecation tend to exceed energy input costs for most plants so it makes sense to run them 24x7x365 rather than over-size them and run them when energy costs are lower.
The energy costs are tied to oil because thats the cheapest option. Well, besides coal. If solar were cheaper wed be using it.
The current record price for solar is slightly over one cent per kWh and still people will write that oil is the cheapest option.
The installation rate for solar is such a steep graph that drawing it legibly is a real problem, see https://nitter.net/pic/orig/media%2FFOoa6xYXIAQKUnv.jpg for example, and still people will write that "if solar […] we'd be using it".