Maybe it preferred Bg5, which defended the knight and developed your bishop? There would be no easy way to remove the knight unless black wanted to trade a rook for knight and bishop.
Edit: disregard this comment. I had forgotten there’s a pawn on f7.
Not easy to see (I am sure I would have played Nxf7 too).
However the logic is clear as soon as you realize that:
17. Nxf7 helps black to develop their h-rook
17. Bf4 develops not only your bishop, but you also prepare development of your a-rook. And you attack the c7 pawn (and with it blacks center). On top of it you keep blacks h-rook undeveloped (and it will not easy to get it out as long as you dominate the e-file, Rae1 is in the air).
This is a good problem for less experienced players, because the solution involves more than just counting material. 1.Nxf7 gets a piece for the trapped knight, but 1.Bf4 Rxr8 2.Rae1 completes White's development and allows him to double rooks and get complete control of the e-file
you will lose the knight no matter what, so stockfish says that developing a piece and bettering your position is worth more than a pawn, especially as it wastes your opponent's move capturing the knight (then you can develop more and get better position than them even if you're even on material)
Another good question: what is the point of posting a useless screenshot? Posting the game's link would have been both easier and more helpful.
The game review says that grabbing a pawn with a trapped night is a mistake (trapped as part of an even material exchange) - I should have just developed the bishop instead. So are there any guidelines as to when to take material and when to develop? (in the game I played Bf4 after black had played Kxf7 - at this point, one move later, Bf4 was also noted as a mistake). Thanks.