Because 4WD cars have a lot of the benefits of RWD cars, in particular good grip on accelerating away from junctions and in the wet due to the rearward weight transfer.
That said you don't need a bloody great heavy brick with huge ground clearance to get the benefits of 4WD. If you're seduced into buying a bloody great heavy brick I don't suppose a few mpg and a couple of seconds 0-60 would bother you.
But hey, it's got bugger all to do with being better, it's all about style and status. Once nearly everyone and their dog has got one (like now perhaps?), the sheep will go looking for something else to demonstrate their "individuality".
Biggles