Theres a vast amount of money in the games industry and having games ported across to other platforms can earn companies a fortune. Look at the revenue of world of warcraft or the sheer volume of sales from modern warefare 2. If you listen to the community and have some talent is pretty damn easy to make a fortune in this area and microsoft understand this.
i wouldn't say its easy to make a fortune.
yes the big titles make a lot of money, but only a small portion of games actually make decent money, a large majority barely break even or fail completly.
there are also so many games that just slip through and no ones ever heard of them.
remember a lot of games cost millions to make, so have to sell a large number just to get the money back, thats y so many games get cancelled, companys go bust all the time, etc