(Strip Number 3680, Original Publication Date, 23 May 1981)
Panel 1: We find ourselves in the kitchen; as John looks on, Elly, who's looking at a sheet of loose leaf, says "Okay, this is my list for Elizabeth's birthday party."
Panel 2: We next find them at the table; Elly tells John that they've got three toddlers, four two year olds, one five year old coming over in addition to Michael.
Panel 3: As he vets the guest list to see if that nice Anthony is coming over, Elly says "We're having peanut butter sandwiches, cheese and celery, birthday cake, jelly, and...."
Panel 4: They both freak out as she says "...ask me why I'm doing this."
Summary: The answer that comes most readily to mind is "Because you'll do anything to make life harder for yourself, idiot." Sadly, both parents are the avatar of a dumb-ass who's convinced that if she let her kids have things they wanted, she'd never be allowed to say no. Also, they're the avatar of a woman who hates birthday parties; we so rarely see them in the strip because they are not only a reminder of her inability to keep Mike, Liz and April's age straight, they're a reminder that she's so much of a lightweight that a bunch of kids can wear her out.
ETA: Now that I've had time to reflect, I'll have to change my answer; since there's no way in Hell Lizzie would care if they didn't have this shin-dig, I'd have to tell her it's because she loves to overburden herself doing needless tasks so she can whine about how bad she has it.