Hartington, it's called m-o-n-e-y. When many airlines want to utilise an airport, as with Heathrow, everyone concerned tries to get the maximum THEY can get out of what's available for the minimum outlay, and who would have it any other way? That's the system (capitalism) we all work in.
The controllers at Heathrow (and the majority of pilots who fly into that port) have it down to a very fine art, to the point where it wasn't all that unusual under the now superceded system to get landing clearance almost in the flare.
The new procedures allow the controller to clear a following aircraft to land with another aircraft still occuppying the runway as long as, in his judgement, the leading aircraft will be clear of the runway by the time the following one lands. (This saves a badly timed radio call from another aircraft preventing the controller from getting a vital landing clearance call in should spacing be that tight.)
In the vast majority of cases, the system works, and works very well. Every now and then, (in a very small percentage of cases), something happens to cause the controller to have a following aircraft go around.
We - the people who fly into Heathrow and other busy airports -aren't attempting to make out we're terribly cool by saying that such incidents are no big deal. In the vast majority of cases, they simply aren't. They make a bit of noise and cost the operator a lot of money - in a 777, for instance, nearly 2.5 tonnes of fuel on average at Heathrow, but given how seldom they do occur, that's a lot less fuel than would be wasted in the long run if we were forced to stretch the separation on landing aircraft to accommodate your suggestion.
Think of it as getting two takeoffs for the price of one ticket, because that's pretty much what a go around is, just another takeoff.