As already observed, probably a habit picked up during training. On a day of high clouds, if flying XC around London I try to fly just high enough to avoid hitting Tall Guy whilst he is trying to avoid hitting Evo. Out over lovely unrestricted East Anglia I lob up to 5 or 6000, partly to allow time if the donkey quits (although see the forced landing thread for alternatives to this) and partly so that the H bit of a HASELL check is already done in case I feel the urge to aerobat en route.
What really baffles me is why so many people doing the cross channel run crawl over the water at 2000 ish feet even on cloudless days.