If you level a city you just removed 95% of the reason for trying to capture that city.

Spending untold billions of dollars and tens of thousands of lives for a pile of ash would never pay itself off.

Israel levelled Gaza after capturing the area, with artillery and with air strikes.

Russia hasn’t captured Kiev, their artillery can’t reach that far and they don’t have air superiority - Russia hardly has an airforce anymore.

Russia fires conventional warheads into Kiev all the time. Thousands of civilians have been killed and injured. The city survives.

Buildings in Kiev aren't made out of wood. Firebombs would do very little damage.

Russia cannot fly a plane into Ukrainian airspace. They barely even fly over Russia's side of the line right now.

They toss bomb from miles back.

Flying conventional bombers over enemy cities requires the ability to replace most of your bombers per year, or air supremacy, neither of which Russia has even a hope of doing.

WW2 was industrialized the likes of which nobody has ever seen again.

Because Russia doesn’t actually have the resources to do this at the scale required. Russia is also not trying to obliterate Ukraine. They want to take over a real economy, not a wasteland.