Nothing really productive comes from that unless you have to. If it only sold 300,000 copies, people would talk about how it bombed and Steam #s were over-rated (thus diminishing the brand).
If numbers are insanely through the roof, the haters can use it as ammo for Valve having a monopoly on the PC industry and how counter-productive it is for the whole eco-system.
Both are extremes, but there's still something to be said for mystery. If you don't have to reveal what your cards are, it doesn't really make sense to show them. It's frustrating for outsiders, but since they have no shareholders to answer to, is probably in their best interests.