Because your are looking at "obvious" things for quality, even though I would argue that Costa when off his game looks like a donkey ...
When you look at Kane, look at a few things
- His balance when he hits the ball
- How few of his shots are significantly wide or high
^^ Those two things are what he does world class, he hits the ball better, consistently more than anyone else in PL (including the two you named), and is genuinely two footed.
Additionally, look at his runs/movement, the goal against Fulham where he went from behind the defender to infront while ball was in flight is a good example.
Kane's best attribute is his striking and ability to hit the ball early and quickly, so in your words, you see the goals, but you are not giving credit to the key capability that gets the goals. His other attribute is his all round game, compare to Aquero (who I do consider a world class striker), Aquero is probably better in tight spaces, dribbling, taking on players, but Kane is vastly better in the air, stronger and physically less vulnerable (yes, when last did Aquero play 15 games in row?), and Kane has very little weakness to his game.
Don't mistake not taking on 3 players every game, and doing flashy stepovers as lack of ability.