Swapping entirely to renewable energy is cool and all but not as easy as “just use a bunch of solar panels???” The issue is that most renewables are some mix of a: unreliable, and b: geographical.

Wind isn’t going to be blowing 24/7 in most places, so wind is unreliable. The sun isn’t always shining in most places, so solar power is unreliable. Hydro is amazing if you have it, but it isn’t the kind of thing you can just build anywhere. Geothermal is also great if you have it, but again isn’t the kind of thing you can just build.

Meanwhile, the power grid requires reliability. It’s incredibly important.

Also, like other people have said: coal/gas are cheap and ubiquitous. Both of those words might as well be synonyms for ‘more money’. Realistically, that’s the primary reason.