Well the DH has changed this a bit, but with the pitcher hitting in the NL, the worst defender got put at first for every team in the league. Does seem silly to have defensive metrics purely against peers when the SS would’ve lit it up defensively at first base but at the same time I got nothing better.
You forgot the main goal of sabermetrics though, it was the way to get nerds paid by teams instead of getting ignored entirely. Just convince them this all means some shit and collect the money. Doesn’t matter if it actually does or not.