There's a couple different reasons. The CADEX rail they are affixed to is screwed into the hand guards, and they often unscrew. The handguards themselves have some play in them, and adding the grip to it makes that 'play' or 'wiggle' even worse. Hardly conducive to a stable shooting platform. And finally, my unit is armoured, the C7 is already too large to be maneuvered in and out of vehicles quickly, the vertical grip whether extended or folded is just another thing to snag on a seatbelt or sling or similar.
In short, those grips were poorly though out and added after the fact, and most serious shooters I know take them off immediately. Now if we had C8 IUR's as our service rifle, a vertical grip would be very welcome as an index point. Personal opinion only, as usual.