Probably for small jobs where WebObjects would be overkill. And they don't use PHP instead of WebObjects they use both PHP and WebOjects, right tool for the job and all that.
They also use SproutCore, jQuery (or have used it), Gianduia, and for the WWDC schedule it seems that they use something else, possibly custom.
[1] http://developer.apple.com/safaridemos/typography.php