The first reason we are asked to clutch Elly to our collective bosom and cry bitter, angry tears about how ill-used she is can be summarized by the sentence "Elly is too God-damned fragile to deal with the kids she has so adding more into the mix is just going to make things worse." The presence of children behaving like children is simply too much for the poor dear to cope with so we can all see how having a bunch of kids acting like a bunch of kids instead of being the grinning little dummies who LOOOOOOOOOOOOOVE sitting until numb, staring into space and never saying anything that Elly needs them to be is a horrible thing.
This leads us into another Foob Fact: Elly's inability to understand any viewpoint that isn't her own combines with her belief that everyone is out to destroy her because they hate her to produce a scenario in which said normal kids act like normal kids because children are evil. They make messes because they hate the idea of her having time to herself, they want to do what other kids do so they can have fun watching her scramble around to cater to their whims, they like presents because they're materialistic monsters who want to make her look like a bad parent and so on and so forth. The idea that they aren't doing things to make her life worse is so alien to her way of thinking that she simply cannot see that she's a jerk worrying herself to death over nothing.
This leads us into Reason Three. The same Elly who thinks that her children are out to destroy her tiny, tiny brain is too blasted dumb to realize that no, there isn't one perfect way to throw a party. She simply cannot conceive of a situation in which she is allowed to not do the same stupid things her cheap-ass mother did so deviations from what she thinks is the norm are evil.
The end result of all of this is that the good time the children have is less important than watching a dumbass wear herself into yet another frazzle over nothing. It's not about Mike being a year older, it's all about Elly feeling a century older and don't you forget it.