Why didn’t Dementors fight with Voldemort in the last battle of Hogwarts?

Question

Why didn’t the Dementors fight with Voldemort in the last battle of Hogwarts?

Answer 1

Dementors did join the battle at Hogwarts.

The Dementors did take part in the fight at Hogwarts. They swarm in a big wave towards Hogwarts when the fight was going on. They didn’t just guard Hogwarts and flee before the battle.

“Shapes moved out in the darkness, swirling figures of concentrated blackness, moving in a great wave towards the castle, their faces hooded and their breath rattling …

Ron and Hermione closed in beside him as the sounds of fighting behind them grew suddenly muted, deadened, because a silence only Dementors could bring was falling thickly through the night …”
_{^{- Harry Potter and the Deathly Hallows, Chapter 32 (The Elder Wand)}}

While Harry fought off the Dementors, the surrounding battle raged on, further proving that the Dementors were present at Hogwarts while the battle was happening.

“It cantered forwards, and now the Dementors scattered in earnest, and immediately the night was mild again, but the sounds of the surrounding battle were loud in his ears.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 32 (The Elder Wand)}}

They also surrounded the Forbidden Forest, where the Dark Lord waited for Harry to surrender.

“He moved on, and now he reached the edge of the Forest, and he stopped.

A swarm of Dementors was gliding amongst the trees; he could feel their chill, and he was not sure he would be able to pass safely through it.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 34 (The Forest Again)}}

Therefore, the Dementors were provably present and fighting for the Dark Lord at the battle of Hogwarts.

Answer 2

They did. Hundreds of dementors fought on the side of Voldemort at the Battle of Hogwarts, though they were primarily employed to guard the gates and to hunt down Harry Potter and his friends. Harry, Ron, and Hermione first encountered them in Hogsmeade:

Then, around the corner, gliding noiselessly, came dementors, ten or more of them, visible because they were of a denser darkness than their surroundings, with their black cloaks and their scabbed and rotting hands.

Aberforth Dumbledore mentioned at that point that the dementors were guarding the entrances to Hogwarts along with the Death Eaters:

You must know they’ve got all the old secret passageways covered at both ends, dementors all around the boundary walls, regular patrols inside the school from what my sources tell me. The place has never been so heavily guarded.

The group encountered them again after making it to Hogwarts:

A hundred dementors were advancing, gliding toward them, sucking their way closer to Harry’s despair, which was like a promise of a feast. . . .

And finally, Harry was attacked as he made his way to the Forbidden Forest and was only saved by the Resurrection Stone summoning the spirits of his loved ones:

A swarm of dementors was gliding amongst the trees; he could feel their chill, and he was not sure he would be able to pass safely through it.