the planes are rather far apart from each other and alumininum isn't the most flammable thing. Also there is about 300-1km cordon around the whole thing with warning signs saying "this is national security area", and it's so sparse that it's probably rather effectively surveiled by about 3-4 people unless, say, 1000 people try to do arson at the same time.

Apparently theft of radar equipment is more of a thing than arson.