That's largely true, but the comparison is a bit of a stretch.
A tablet (or a smartphone, or a laptop) can be made indefinitely useful with just a handful of applications. By contrast, a game console needs a constant stream of titles being ported to it or it will soon become as useful as a doorstop.
Also, if you want to - say - browse the web, it doesn't matter much which browser your tablet provides, as long as it does a decent job. But if you want to play some specific game, no substitute will ever do.
Bottom line: the Surface doesn't have it easy, but that's a walk in the park as compared to a new game console.
There are also advantages that game consoles have over tablets; take the Wii for example. It had very few good titles for the first year or two but yet sold like hotcakes. It resonated with users beyond what the market had defined as a "game console". SteamBox has the same opportunity. It doesn't mean they'll be successful. Just looks at Microsoft; the Zune, the Kin, and the Windows Phone. Just because your "PC" can run more diverse software doesn't mean it will resonate with users.