why isn’t this a checkmate?

If you mean the following game, it's black to move, they are not in check, and they have no legal moves. That's a draw by stalemate.
https://www.chess.com/game/live/137720388088
If you mean the following game, it's black to move, they are not in check, and they have no legal moves. That's a draw by stalemate.
https://www.chess.com/game/live/137720388088