Trout gets more pitches in the strike zone and walks more because his discipline is much better than Harper. Trout is 20.4%, 9th in MLB in laying off outside pitches (O-swing%), while Harper is 28.6%, 68th in MLB. Yes, he has a discipline problem even though he's drawing walks. he is patient (4.08 p/pa, well above average of 3.85), but not disciplined.
Betts, BTW, is 8th in MLB in laying off outside stuff at 20.2% O-Swing. Freeman is near Bryce (28.5%). Altuve, who is a completely different hitter, is at 35.2%, but is allergic to walks and incredible at contact.