Because Vaughn and Moncada are vastly better players than Sheets and Burger. AV could well be the next cornerstone of this franchise; I don't see the Sox pinch hitting for him late in games, nor should they. And by this pinch hitting role you're putting Burger/Sheets in, you're also downgrading the defense substantially for the rest of the game for what amounts to maybe a very small matchup advantage in 1 at bat, that in many scenarios will be counteracted with an immediate pitching change anyway. I just don't really think Pedro is going to manage that way - he is going to stick with his core guys late in games, IMO.
Look, I like Burger and Sheets. They are fantastic depth pieces. Both are likely to get a lot of MLB at bats at certain times in 2023 due to injuries. IMO, they should both start the season in Charlotte getting everyday at bats and every day reps in the field assuming Colas breaks camp with the Sox. IMO, the bench SHOULD be Seby, Leury, Alberto and a RHH OF that is a plus defender via a waiver claim / milb deal guy that doesn't make their current roster (or I guess one of the internal OF options (Billy/Marisnick/Haseley) if nothing materializes there). I think the bench WILL be Seby, Sheets, Alberto and Leury. The only way Burger makes a single lick of sense on this roster on OD if is Colas is in AAA.