Because they will not help a new player as a whole. The cards are jank and have no play ability against anything other than other plainswalker decks. The decks are poorly built. They don't provide a good launching point. Honestly I could build a better single expansion all common deck. In fact I build such decks fairly often.
When I see a new player I teach them with the free decks wizard provides and if they like the game I help them build a deck out of draft leftovers. These decks would retail for less than those plainswalker decks had the bought the cards as singles, but are better decks, with better cards. Yes they will still lose a lot but at least CAN win as the deck is at least built right with a good curve. And upgrading is easier as the deck has a better base.
I have taught over thirty people how to play in in my life. Most still playing to this day. My advice on these products is to avoid most of them. Play the starter deck I give them. Update ones collection though draft, prereleases and singles. Well and Commander decks as they often have good value and are playable out of the box if they want to start playing commander.
I would never suggest anyone buy a painswalker deck outside of collectors wanting to own them for completeness sake. They are just a bad product. Poorly built, poor card quality, and poor value. There is no upside for them to start the game with these weak decks that aren't a real upgrade to the free decks wizards gives newbies. I am with Azurhawk, my advice to new players is not to buy those decks. They aren't a good starting point. They don't demonstrate how to build a good deck. They are needlessly bad and aren't a good product for anyone really.