especially since Frank Robinson was an outfielder.
You probably meant Brooks Robinson. Same last name and team, likely no relation.
dWAR is absurd. Don Mattingly saved SO MANY errors and infield hits as a first baseman who would nail splits that would make a Russian dancer jealous. All-time record high fielding percentage. Lightning reflexes. Literally by far the greatest fielding first baseman of my lifetime. And a negative dWAR.
Greg Nettles has been called “at least the equal” of Frank Robinson at third base, admittedly by Yankee-fan announcers. I never saw Frank, but Greg was the best I ever saw. Also a negative dWAR in 1980, 82, 83.
Better than Brookes Robinson? Not even close. Not even good enough to be in the top 10.
The dWAR figures are adjusted for position. First basemen as a rule have low dWAR ratings because a great defensive player at that position isn’t nearly as valuable as a great defensive player at any other position. Look at a list of the top players of all time as measured by dWAR, and you’ll find that most of those at the top of the list are shortstops, third basemen and catchers.