They're probably too scared to make any more half-life games because of the expectations that fanboys (me included) have. At least we don't expect LFD3, TF2/3, or Portal 2 to be life-changingly spectacular.... except for maybe Portal 2....
Also, Valve is known for playtesting the CRAP out of EVERYTHING. If something goes awry in playtesting they re-vamp the whole game because Valve fans expect nothing less than a near-perfect gameplay experience. But the games are worth the wait, and from what I can tell from interviews and director's commentaries, the extensive playtesting is a huge reason why Valve games perform so well.