Why didn't Sauron make his hand whole?

Question

In The Fellowship of the Ring, "The Shadow of the Past", Gandalf tells Frodo:

Isildur Elendil’s son cut the Ring from Sauron’s hand and took it for his own. Then Sauron was vanquished and his spirit fled and was hidden for long years, until his shadow took shape again in Mirkwood.

So we know Sauron didn't have a body until he took shape again. But we also know, from The Two Towers, "The Black Gate is Closed", Gollum says:

Yes, He has only four on the Black Hand, but they are enough.

If I were an immortal being who could eventually form a new body when the old one is destroyed (which, to be sure, I'm not) I would create a body without such a flaw as a missing finger, especially one missing as a reminder of how I last lost. Maybe make something completely different, like a prehensile tentacle or something.

I have read The Silmarillion in addition to The Hobbit and The Lord of the Rings, and recognize there may be insufficient detail on how the Ainur can take physical form, but wondered if there was anything perhaps in other writings, like letters, that may shed light on this.

Answer 1

He was not able to do so. In the "Akallabêth," it is clear that Sauron (at least after the destruction of the fair body that he wore as Annatar and in Númenor) does not have the power to alter his form freely.

But Sauron was not of mortal flesh, and though he was robbed now of that shape in which he had wrought so great an evil, so that he could never again appear fair to the eyes of Men, yet his spirit arose out of the deep and passed as a shadow and a black wind over the sea, and came back to Middle-earth and to Mordor that was his home.

This mirrors what happened to his master, Morgoth. After the Darkening of Valinor, he took the form of a dark lord, tall and terrible—which he was seemingly never able to leave. After Morgoth's duel with Fingolfin, his foot was permanently maimed. As the dark lords extended their power over Arda, their innate abilities as Ainur were dissipated and diminished.