Because sitting in cover wins lots of games? Especially when you only get one overpriced special weapon every ten guys. You lose with kill points, and you don't have the mobility to take objectives. Plus, even with FNP warriors drop like flies.

Sitting back lets them prioritize you. Charging at their face sends them into an unplanned panic. I have never lost all of my vehicles playing and form of guard. Once you get in their face you multi-assault Wracks on all the guys that fell out of the popped transports. Between darklance damages and easy kills of their troops, they are at half strength at turn one, plus they have no strategy to counter, plus even if they did, you have so much more mobility the rest of the game to do what you need to do.