There is a difference between drawing walks and being selective. The latter can lead to the former, but not necessarily.
Some hitters, like Stott, should draw a lot of walks b/c he's such a good 2 strike hitter he can take borderline pitches and live with calls going against him. So it makes sense for him to be patient with less than 2 strikes and look for a pitch he can drive. Hitters like Schwarber and Haper can take pitches b/c they hit with so much power than you accept the Ks as part of the package.
Others, it's more a matter of getting a pitch you can hit instead of wasting strikes on pitches you have no chance to hit. If you don't chase bad pitches, odds are you'll get one or two per PA in your "sweet" zone.
Castellanos is going to fall off the cliff at some point b/c he "wastes" too many strikes, as his bat speed declines with age, his aggressive approach will set him up for failure. He's also "pull happy" and turns too many outside pitches into GBs to SS. Compare to Bohm, who has learned not to force it, he's gotten better at turning on inside pitches while taking outside pitches to the other field.