(Strip Number 6090, Original Publication Date, 3 October 1982)
Panel 1: We start things off watching the kids having lunch; as Mike eats his Kraft Diner, Lizzie frowns and pokes at the salty orange lump in distaste.
Panel 2: The only thing about it that does make her happy is that it makes a PTOIIIIIING noise when she flips it in the air.
Panel 3: Elly's disembodied hand points at Lizzie's plate and her disembodied head tells Lizzie to eat her lunch; Lizzie glumly replies "No like macamoni."
Panel 4: Since Elly doesn't remember the past very well, she's baffled by the fact that Lizzie balks when she tells her "Come on....open up" and tries to force-feed her.
Panel 5: Lizzie's angry SHRIEK that she NO LIKE MACAMONI!! really confuses Elly because, as we know, Elly will eat anything that doesn't put up too much of a struggle. Since she can any thing, she doesn't understand that her children hate certain foods.
Panel 6: Since Mike is a growing boy and since he probably wants to score points with Mommy, he asks if he can have Lizzie's lunch if she doesn't want it.
Panel 7: Elly's handing the plate over to someone who'll actually eat it causes Lizzie to SHRIEK "MY DINNER!! MINE!!"
Panel 8: Since Elly thinks that Mike's trying to take it away from her and her reaction thereto has settled things, she tells Lizzie to eat her lunch.
Panel 9: Lizzie's telling her that she no like macamoni baffles Elly so badly, she looks like she got hit with a brick.
Summary: This is because she never saw a child not want to play with something but also not want someone else to have it. I don't quite know why children do that but I do know that this need to play dog in the manger is a fairly commonplace thing and thus should not surprise anyone who claims to know children.