Because otherwise, all my toys would die.
I had bought into the logic of the Jim Henson Christmas special The Christmas Toy, which shows us that children's toys love to play and lead their own active lives while human beings aren't around. Unlike its descendant Toy Story, there's a dark twist: if human beings catch a toy while it's being "alive," then that toy becomes frozen forever, which is pretty much depicted as toy death. (Would a child be able to tell if the toy he or she was playing with was already dead? The mind boggles.)
Rugby the stuffed tiger was Jamie's favorite present last year, so this year Rugby assumes Jamie will want to open a present and find him inside again this year. He tries to "regift" himself without considering that if Jamie sees Rugby after he moved himself, Rugby will die. Some valiant toys try to save Rugby and are killed in the process. If you're not sobbing at the funeral procession for Ditz the clown only about 10 minutes in, you'll certainly be bawling by the time Mew the cat toy, a heartrending victim of toy racism, lies dead on the carpet while Rugby sings his dirge.
If your mother finds a toy that is not where it should be, it is not because you failed to pick up your things, it is because the toy was out playing and now she has just killed it.
The Christmas Toy. Dir. Eric Hill. Perf. Dave Goelz, Steve Whitmire, Kathryn Mullen. The Jim Henson Company, 1986.
The Christmas Toy
(See it on YouTube here)