They invest into developing a game and get all the sales within their own game store, plus the exclusivity boosts the sales of the console itself and next titles in the franchise. Then, after all this effort and cost is long gone, they get to invest minimal effort to sell and hype the same game all over again to another audience and, again, try to hook these players into the franchise perhaps even selling a console to them. There are no downsides, only extra money.