a touchback gives the team the ball on the 25. A high kick that comes down at the goal line (while allowing the risk of a nice return) typically results in the opponent starting the drive inside the 20. That's one more first down they have to make to sustain a drive into points and coaches care about that. The only reason to just pound the ball into the EZ on every kickoff is if your D is so stout that you just don't take the risk of a mistake on kick coverage letting them house it on you.