You're ignoring the physiological 'cost' of knowing some filthy stranger will be poking around your nice house. Also renting out a place means you cannot store anything of value in the place. For many people who can afford top end apartments in central London, those two 'costs' will be much higher than any income they might get from renting.
Also the value of having a flat like this is knowing that when business requires you to fly to London on 90 minutes notice, at least you'll have somewhere nice and familiar to crash. That is completely lost if there is a chance the someone else might be staying in your apartment when you arrive