Resign vs keep playing


They resign because it's pointless. If you are up a full minor piece, it is basically always an easy win. For example, consider the position below.
White can win by simply teaming up on the black pawns with the king and knight, but he can also win by sacrificing the knight for a pawn, which would probably be a winning king & pawn ending. Once you get to a certain level, up two pawns or a piece==automatic win. Being a rook down is always losing. You can test this by setting up the starting position against the bot and remove a rook from the computer's side. You will probably win pretty easily. People resign because they have better stuff to do and they know that their opponent will not stalemate them. Hope this helps!