Um, because they aren't all fancigarettes and play on multiple platforms?
1. Why would many PSN and Nintendo Wi-Fi users have steam?
2. Why is Xbox excluded from that goup?
3. Some facts I found:
As of June 2010, there are now over 50 million registered PlayStation Network accounts worldwide.
On February 5, 2010, Marc Whitten announced that Xbox Live had reached 23 million members.
Now obviously there aren't 50 million PSN players, it is more like 20 million as the accepted stat, but no one can really be sure.
Nintendo Wi-Fi doesn't have accounts, so that can't be definitively accounted for.
Nonetheless, more people have accounts on console than Steam accounts on PC.