Because some 5,000,000 10,000,000 are bored and each of those people are trying to access somtimes 2 and 3 different sites simaltaniously, and when the sites don't load they keep hiting the refress button.
Now I haven't looked to see what sites get most of the traffic or to which of the server farms most of the traffic is going. But I can bet with that many people "surfing" every ISP, domain router, and server farm are see a hugh spike in traffic and I would bet most of the IT guys can't figure out why.
Just to note 10,000,000 is 5% of all internet users in the US and if you calculate that out most server farms don't see that much traffic in a week. It is a massive strain on the internet as a whole for that many people to sundenly be surfing instead of using blizzards servers.
Combine that with the fact that most people trying to use the World of Warcraft website on patch/maintenance days are looking for 2 piec of information. Is my server up? Or when the hell will it be up? Since Chops added server status I'll be he is taking a hit since people can now go to their guild site and look at the status of their gaming server there and not deal with Blizzards site being bombarded by the internet newbs that Krik describes so elegantly...LOL
