I do understand where you're coming from. However, As stated before, there were specials in which adults were seen and heard, cases in point, "Bon Voyage, Charlie Brown", "It's Flashbeagle, Charlie Brown", "This is America, Charlie Brown", "Snoopy's Reunion", "It's the Pied Piper, Charlie Brown".
Another point as to why I would like to profile the lives of the adults of Peanuts is because Charlie Brown's misfortune would more than likely require a strong and compassionate mom and dad to help make his life more tolerable. let me list some examples.
Charlie getting a bag full of rocks when trick or treating in "Great Pumpkin". Mr. and Mrs. Brown would be shocked and appalled by the actions of those people, and one instinct would be to take those rocks and throw them through the windows of the houses who shafted their son for no good reason at all.
Another would be when Lucy constantly pulled the football away at the Homecoming Game in "It's Your First Kiss" deliberately costing their team the game and scapegoating Charlie. Adults wouldn't be stupid. Mr. and Mrs. Brown, and especially Mr. and Mrs. Van Pelt would have noticed this and would have called Lucy out on it and had her punished for her cruel trick.
How about in "Happy New Year, Charlie Brown" when his teacher made Charlie read the entirety of "War and Peace" over the holiday and then deliberately flunk him by saying she doesn't believe he managed to do so. Mom and Dad Brown would have unleashed their wrath on that teacher that words couldn't describe.
There's nothing wrong with Schulz's decision to make it all about the kids and how they handle their problems themselves. But it doesn't mean that an adult perspective couldn't be explored.