The game is constantly evolving/changing. With the amount of film/video/data teams have these days, teams can implement a strategy to contain opposing offenses. Opposing teams and individual players do not sit idly wondering why they can't get the best of another team/athlete.
So the issue is that Kane's game has not evolved and/or adapted to the changing dynamics of the game. You'll see a lot of the top athletes in the world change their game up and figure out how to maintain an elite level. As an example, look no further than Ronaldo - up until last year, he was one of the top players of the game despite declining physical abilities. His game has evolved considerably throughout the years thus has consistently been able to play at an elite level. Same thing with Michael Jordan - his game evolved and became a completely different player during his championship run during the latter years of his career.
So the solution? If Kane's game doesn't adapt, the solution is that we should only start Kane when it tactically makes sense. He should not start every game without thought. We need to implement some sort of rotation to keep our offense fresh and unpredictable. Our attack has become too predictable with Kane up front. Opposing defenses have evolved and seemed to have figured out how to contain Kane. If we're able to give opposing teams different looks with different players up front, they're going to think twice when they're going through their own film/training sessions. We have to firepower to execute this.