To be fair on fossil fuels, they are simply stored energy from the sun. You can think of them like a dense battery, more dense than our current battery technology allows.
The former is actually continuous, and thus far more reliable. The latter requires coming up with some other storage mechanism. Granted, we have ways to do this already. But it's still not a trivial project.
a dense battery with too much side effects (fumes, CO_2/etc. gases)
a less dense battery with much less side effects.
Solar is so diffuse, just bringing it to where people need it has doubled the price purely in transmission infrastructure costs.
Reference: Australia - the place that’s supposed to be solar’s poster child has more than doubled electricity prices in the last three to four years because, unsurprisingly (we were warned), getting solar and wind to where they’re needed turns out to be incredibly expensive.