Not every country has access to a desert to deploy large scale solar. Most of the EU, Russia for instance, and those places do not want to be dependent on other countries.
It's probably also worth noting that photovoltaics have to be exposed to the sun to work... which means that they can't be protected by armor or bunkers, which means that they're terrifically vulnerable to terrorist or other attack.
The terrorist attack that is going to take out 1000s of square kilometres of solar panels? Surely they'd just attack a city if they had that amount of firepower? Or in the more likely case that they don't it's easier to attack power transmission than generation.
Assuming there are that many panels, it is in fact quite easy for any industrialized country to design a weapon that can destroy a square mile of panels or more with a single warhead. They're thin sheets of silicon covered in glass.
A square mile is 2.6 square km. You are replying to someone who said it would be hard to destroy thousands of square km of panels. It would seem that your reply, that "it is…easy…to…destroy a square mile of panels or more with a single warhead", is not to the point.
However, by coincidence, your position is right, for two reasons:
1. Single nuclear warheads routinely have blast radii of tens of km rather than, as you suggest, hundreds of meters. So in fact a single warhead can indeed destroy thousands of square km.
2. 1000km² of solar panels would be 1000 GWp; at a low-cost module price of €0.19/W (the average for 2019) that's €190 billion. Currently solar plants are not built that large, nor nearly so.
I was replying to a comment that said solar panels are vulnerable to terrorists.
If terrorists have a nuclear warhead they are going to use it against a city not solar panels. Similarly if terrorists have a weapon that can take out thousands of square km of panels it can easily take out existing powerplants (which btw are not armored or in bunkers - apart from some limited shield on nuclear powerplant cores.
The point here is that solar panels are not more vulnerable to terrorists than the existing infrastructure and are probably less vulnerable due to size and how distributed they are.
Most militaries rely on oil and dirty fuel to run campaigns. There's mandatory fuel stockpiling. The event of a power station going down would be seen as an act of war and suddenly there's much bigger problems for everyone involved to worry about.