Er; bravado, foolishness, excitement, boredom, showing off, stupidity, fun, because they can, lack of imagination, too much imagination, willy-waving, knowing they are unlikely to be caught, I could go on.
But not, I think, inaccurate instruments. After all, what instruments are involved in the low bit of low flying apart from Mk1 eyeball and a brain? Mmm, perhaps that's the problem...