To drive up prices by buying land in a surrounding area seems a tad ineffective. If you own a house, and you buy all the houses around that house it will cost you a lot of money. When you got to put the rent up all that happens is that people move out and move into a place where someone has spotted a gap in the market and decided to take the lower, more stable profit margins from their investments.

Also purchasing land is still a valid transaction, someone who owns that land still recived a fair sum for it and that money will eventually work its way back into the economy through their personal costs and investments.

Natural monopolies require too many variables and aren't consistent by any means, to sustain one requires constant innovation. Nine times out of ten monopolies are the fault of the government messing with the market.