Because I did not add advanced Burning Boxes for Liquids and Gasses yet, that is why. And they are planned, so that is why I "ignore" your complaints simply because something else I plan will fix that particular issue later. And I know people who still use Solid Fuels for Stuff so that argument of yours is not fully valid either.
And they can use/produce 50 units of solid fuel per minute?
Even in present mankind still using solid fuel for genereting energy => coal powerplant, or produce heat in some houses. So this tech isnt obsolete
So? I'm talking in game, not real life. In game, fluid fuels offer many advantages over solid so there is very good reason to switch to using them as soon as you can. And once you start refining oil, you find yourself in possession of large amounts of them.