They are, as is every statistic, opportunistic.
Aside from the rare bases loaded walk/HBP, you can't get RBIs unless you swing the bat.
Kepler, Polanco, and Cruz, often hit in front of Rosario, and often in that order. They all have good OBP, all higher than Rosario's. Scenario: Kepler draws a walk to lead off. Polanco hits a weak ground ball, getting thrown out but advancing Kepler to second. Cruz strikes out. Rosario hits a single. Kepler scores. Sano Strikes out. Inning over, RUN SCORED. Flip Kepler and Rosario- same scenario, same individual production for each player. Rosario singles, advances on Polonco's out. Cruz Ks. Kepler walks. Sano Ks. Inning over, NO RUN SCORED.
It's just one scenario, right. But it exemplifies the value of the single in comparison to the walk depending on lineup construction. The best a walk can be is equal to a single, but often, a single is far greater than a walk. Kepler's walk total is comparable to Rosario's singles total. If Rosario traded 20 of his singles for 30 walks, it would be a significant bump to his OBP, but his RBI total goes down, probably under 100.
Taking a walk when given it, and having a measured, consistent approach to batting are valuable to a team's success, undoubtedly. These are not a part of Rosario's game. Everyone crushed Mauer, wrongly, for the duration of his career for being one of the best ever at practicing such an approach. He got crushed for not being willing to guess or expand or surprise. Mauer is an all-time Twins great, but he wasn't willing (in his approach to batting) to take a risk or a chance to be a hero. He wasn't willing to gamble on his own natural ability as a hitter in favor of doing the right thing as a batter. I'm a fan of Joe. I loved watching him play. I wish he was in the dugout/on the field for the 2019 run. But, there were many times when I was ready to lose my mind watching Joe take a walk with a runner on when we needed an RBI. And that doesn't mean he was wrong. It does mean I'm happy to have Eddie up there going after that RBI, even when he ends up looking like a fool.
It's pretty hard to strike a balance between patience and aggression. I mean, they are pretty antonymous. Most guys are going to sacrifice a fair bit of one for the other. The guys that have it balanced, the best of both, are Mike Trout. So if Rosario could add 20 walks to his 2019 total, without losing any singles, he'd be even higher on that list of MVP candidates, he'd be a lot closer to Mike Trout.