[Petronski]

So why would anyone who follows Christ, who is in Christ, and who is indwelt by Christ through the Holy Spirit need a human to be a vicar of Christ?

Because Christ appointed us one.

[marajade]

"Because Christ appointed us one."

No because God appointed us one, and it was Christ.

[DouglasKC]

So why would anyone who follows Christ, who is in Christ, and who is indwelt by Christ through the Holy Spirit need a human to be a vicar of Christ?
Because Christ appointed us one.

1Ti 2:5 For there is one God, and one mediator between God and men, the man Christ Jesus;

That doesn't leave much wiggle room for anyone else.