Why is MS not making any exclusive games for WP?

New platforms live and die by exclusives. Considering #1 weakness of WP is lack of apps and games, why is MS not pumping out more amazing games for WP? Instead they seem to be focusing more on other platforms? Did they not learn from Xbox and Halo? Xbox wouldn't have taken off like it did without Halo as an exclusive.

Us WP folks need something to shout about, and MS is not doing a great job of giving us the ammunition.