Because gobject is such a pleasant and C-like system to use.
(Seriously. I like most of glib, though it is a bit memory-inefficient for some uses -- but gobject is a classic example of overgeneralization yielding a system that, while quite general, is terrifically difficult to use for almost all practical use cases. This is a fairly common trap to fall into, so I'm not *blaming* anyone, or I'd have to blame myself for several even more baroque systems I've perpetrated in the past which thankfully have not got into the wide world. But still.)