There are multiplayer games, you just have to look for them. I'm just going to assume you meant, why there aren't more multiplayer games.
Well the reason is simple. The devs don't want to make one and create games that they are comfortable and interested in.
And no, AG3 is a revamping of the site and site features generally, not one where they make major edits to the games. So I can't answer you directly. Will there be more multiplayer games submitted after AG3 rolls out? Maybe. Maybe not.
There is multiplayer games you just need to find them and look harder. Though it would be nice if there was a tab for showing which were the multiplayer games or not.
I think a multiplayer game tab would be useful, but I'm am wondering if there are any local multiplayer games, I mean games for 2 people on the same computer and if not, why?
A simple way to find multiplayer games on ArmorGames is by using a search for 'multiplayer' in the search box on the left top corner... with this for a result!