Because people are thinking that it was played backwards to Kane who they think is onside.
I've seen people try to argue that the defender was deliberately playing the ball to discount Kane's position, though I agree that it wasn't deliberate, but I've personally not seen anybody saying that it doesn't matter if Kane was offside because it was passed backwards.
I've only seen people saying that Kane wasn't offside because the ball was passed back to him.
I'm not in the country right now, so I've only seen the text comments, maybe people were saying it on BT broadcast last night, in which case that's fair enough in calling them out.
But that's nothing to do with the VAR decision, which was that Kane was always ahead of the ball.
No idea why it took them 5 minutes to decide.