Off the top of my head, Alfonso Soriano, Dante Bichette, and Adam Dunn would strongly disagree with you. It is pretty simple to be bad outfielder, I'll grant you that. Hanley Ramirez, circa 2015... Aside from Hanley, those guys just weren't athletic guys, they could hit the ball and had to play somewhere in the field. No amount of practice was going to make Adam Dunn a better outfielder. He just didn't have the physical tools to be good at it. You get better in the OF by playing it. There just really isn't much to practice. With infield play, the repetition of taking ground balls, turning double plays, knowing where/when to take cuts from the OF, cover bases, look back runners, and follow trail runners are things that you can get better at with practice.