First, Apple does not have a monopoly, but rather vertical integration. These are different things.
But, secondly, as others have pointed out, Apple stands to make more money on selling computers and OSX than just OSX.
If Apple allowed OSX to run on other PCs, the company would lose some or all of it's hardware business (and the fat profit margins therein) while dramatically expanding the development costs of OSX. Now, Apple would have to worry about not just having Leopard work on a MacBook Pro, but also dozens of other variations in motherboards, chip designs, custom WiFi setups, and other little variations that exist in PC land.
Yes, exactly. Apple got you to pay for a new computer and OSX, and all they had to give you was Bootcamp. Rather than develop for a whole slew of PCs, they just had to create this capability. This is a good business decision.
Because the walls of Microsoft are decorated with the skulls of those who've tried before.
Actually, Apple has a fine line to walk, they want to develop iWork into a viable Office replacement, but they want to do so in a way that won't destroy the Mac Business Unit because a vast majority of people in PC-land and on the Mac use Office. In fact, most places including businesses and universities (not to mention the legions of home users) have MS Office and thus would be hesitant to use OSX if they would run into compatibility problems.