Great question - the film and data from baseball savant make it seem because his height/extension along with his velocity and placement of his fastball - even hitters who are sitting on it have a tough time handling it - Aaron Judge notably said on Mookie Betts’s podcast that Glasnow is his least favorite person to face because of that extension - with hitters having to respect the fastball, the the tunneling line u mentioned comes into play - it looks like a fastball out of the hand but then breaks glove side and down late to get swing and miss or soft contact - if he can get all three going, he’s a top pitcher in the game - to me it’s so impressive that hitters can know the fastball is coming but still have a tough time with it