그러다 stands in for whatever behaviour was just mentioned, and the 니까 clause gives its reason, so the whole thing explains or excuses someone: they are like that because we have been in the business so long, because the kids are young, because your blood sugar spikes, because it is not a normal bakery. It is the standard way of accounting for behaviour you are not going to describe again. One line uses it to complain that advice keeps telling you to do the very thing you cannot do.