In most cases, it’s not worth the extra effort to cast it nonverbally.
It's made clear that casting the same spells nonverbally takes much more effort than it would if the person casting it says the words, even for a powerful wizard like Lord Voldemort. Nonverbal spells are consistently said to be much harder to cast than their spoken counterparts.
The first quote that I could think of mentioning the difficulty of nonverbal spells is this one from when Snape was teaching them in Harry's sixth year:
“Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some ...lack.”
- Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)
In most cases, if Lord Voldemort is casting a Killing Curse at someone, they're soon to be dead. When fairly sure that his target will be dead soon, it wouldn't make sense for him to waste the energy on casting Avada Kedavra as a nonverbal spell.
He can indeed cast it nonverbally - he casts it nonverbally at the Ministry several times when dueling Dumbledore. This makes sense, as Dumbledore is a powerful opponent he’d want to give himself every advantage he possibly could against. Dumbledore was very skilled and the only one the Dark Lord ever feared, he wasn’t going to take a duel with him lightly.
“But even as he shouted, another jet of green light flew at Dumbledore from Voldemort’s wand and the snake struck –
Fawkes swooped down in front of Dumbledore, opened his beak wide and swallowed the jet of green light whole: he burst into flame and fell to the floor, small, wrinkled and flightless.”
- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)
Just because he can do it nonverbally doesn't mean it's always the best choice. In most cases where he says the words Avada Kedavra, it's not while facing someone like Dumbledore. It's when he's using it on people who don't have much of a chance against him that he says the words. For example, when he killed the Potters, he said the words because the targets of his curse weren't going to be able to stop it, even if they knew it was coming.