Same as mentioned, its more for the social circle and connections. They high net worth got more things in common. Also all these events and such they can match each others.
Nowadays think kindergartens, especially quite some private ones perhaps, tend celebrate the kid birthday in class with the rest of the classmate. So, have to spend extra on the cake and gift. If not siam on that day, which the kid might feel left out.
Also, those major festive holidays the kindergarten tend celebrate with the kids also, so as to introduce the kids into such festive. Not sure about gift, but think clothing have to spend.
And think for the elites, they may tend have their own private party and gathering for the kids and thus parent to come together.
Thus if squeeze into the elite circle kindergarten, and want to network with them, best be ready got that extra moolah to keep up bah.