I don't even really like going there, which is why I kept that comment short.
To Phil's credit, I think it is partially because he's too much of a nice guy, yes. I think he saw Randy Sanders as something of an apprentice back in the day. He still believes he was done wrong/railroaded by the admin when he was fired, and is really hesitant to do that to Pruitt, who he hand-picked.
However, he also doesn't want to let him go because he is so wedded to Pruitt at this point. Phil was brought in under an incredibly unique circumstance; the AD was fired, he was installed, and immediately had to hire a football coach. He said "the Vols are back" at some banquet last year. He's connected at the hip to Pruitt and probably thinks that letting him go makes him look really bad too.