I think the main reason Scalia or Thomas wasn’t elevated is because this would have created a separate and contentious vote. It was far easier just to appoint Bob Roberts as Chief.
Good point, it forces the President to try and get two nominees thru the Senate at once, when he only needed one. Oddly enough, I believe Scalia was confirmed 96-0 in 1986, which would never happen in today’s hyper partisan climate.