Because they can! What can we do about it?
I can't remember the last time I actually had time to visit the lounge anyway. It's normally a 2 km sprint to the gate, with kids, and no baggage on arrival..
If you ever do try to stick the kids down the back to make room for a mate then all that will happen is you or your wife will have to go back there as well to look after them(unless you craftally booked them as unaccompanied, and totally ignored them at checkin) and then the next in line for the upgrade will get those seats. That could well be a frequent flyer upgrade or full fare eccon pax etc etc, you or the guy behind the desk has no say in the matter.