Both AG and FBI Director are confirmed by the Senate. Trump could appoint anyone he likes, but would McConnell have confirmed him? This is how the deep state operates. And This is how Trump wound up from the start of his Administration, with the treacherous Jeff Sessions, who immediately chained him down for two years with "Russiagate."
“... the treacherous Jeff Sessions...”
And herein lies a tale? Who would have even known during the campaign in 2016 that Sessions would prove so weak, even suspected of treachery? He was often at Trump rallies being pointed out for praise. How is it possible to know who to trust in this environment?