Character mistake: When Kevin is shooting the toys off the laundry chute with Buzz's BB gun, he's looking through the scope with his closed eye.
Character mistake: When the family comes home, Kevin says he went shopping and bought milk, eggs, and fabric softener. We never seen him unload the eggs onto the conveyer belt at the store nor did we see the check-out lady scan them.
Answer: I think it was a way to make leaving him "home alone" more realistic and understandable as opposed to absurd. Being perceived as a brat/pest and annoying to be around, it is (somewhat) conceivable that none of the family members would be eager to have Kevin by their side. This "frees" all of them from noticing that Kevin isn't with them. Everyone would just assume that Kevin is somewhere among them and each be glad they didn't have to sit next to him on the way to the airport or during the long flight.
KeyZOid
In addition to this, the movie is partially about Kevin learning to have more respect for others. He appreciates his family more as he spends more time without them.