Why do the worst fires occur at oil refineries?

It may be an unsupportable assumption that the worst fires occur at oil refineries. Oil refinery fires are usually extinguished quickly and are usually well contained.

It all depends how you define 'worst'.

The most difficult to stop fires tend to be fires in forested lands. The most expensive fires tend to be large commercial buildings. Fires with the highest human death rates tend to be in dense informal urban settlements - so called slums. The most polluting fires tend to be associated with gas flares and well fires in oilfields.