There's just one problem: they take FOREVER. I like socializing as much as the next person but most of us are hardworking people and we need our shabbat nap. We might all stay up late chatting but then we have to drag ourselves out of bed the next day to make it to shul on time.
The real issue is the lack of a defined end point for the meal. No one wants to end it. The hosts don't want to feel like they are pushing their guests out so they hesitate on handing out the benchers, or serve dessert after benching. The guests don't want to 'eat and run' yet don't want to impose on their hosts either. Besides, maybe some of the single people are liking each other or maybe some of the married people are thinking of shidduchim for these single people. You don't want to ruin someone's shidduch because you need a nap.
Why not just leave? Because once somebody makes a move everyone will follow. No one wants to be responsible for that.
Of course it could be worse. I hear in Washington Heights they end up going straight from the meal back to shul. That's practically a six hour meal. Plus you'll never be dairy after shabbat.
May I am just getting old. Perhaps four to five hour meals are normal. But I still think if you can't eat it in two hours, maybe that's too much for you to be eating anyway.