Every Senator confirmed to the Cabinet is one less Republican Senator in the Senate. It could also open up a special election that the Democrats could win. Right now that could cost the majority in the Senate.
It makes thing harder for the GOP in the Senate, which they won’t appreciate.
Trump could go oustside the Senate, but everyone he seems to know has that air of: “Are you fucking serious?!”
“Hi, I am Ivan Notrussian. I am Comrade… Er… President Trump’s new AG nominee.”