As another commenter said, this is probably partly down to cultural expectations. The ideal would be to sit and eat together. In reality, one family might get held by up to an hour because their baby napped later than normal; another family is half an hour late because of traffic; another family had an nightmare nappy blowout situation and has to go back home for new clothes etc... . Being relaxed about arrival times is less stressful all round. The children will still socialise with each other in the overlapping times they are together.