Even if he didn't want to use them, why didn't Voldemort steal all of the Time Turners?

Question

This question covers why he wouldn't want to steal one and use it, but that doesn't explain why he wouldn't want to steal them all anyway. Regardless of if you plan on using it, simultaneously gaining access to (maybe risky) time travel and forbidding it to your enemies is a pretty sweet deal. So, assuming that he knew where to look for them, why didn't send someone to grab all of them during the events that take place in the ministry during the fifth book?

Answer 1

It would have been a waste of effort.

Voldemort had several things that he and his Death Eaters needed to do to achieve his goals of immortality and world domination - such as gaining more followers, infiltrating the mind, devising a way to kill Harry Potter. Finding and stealing the Time-Turners would be a waste of their time and effort. Stealing them so his enemies would not have them would not give him much of an advantage, because they are of very limited use. First of all, changing time is breaking one of the most important wizarding laws, so his enemies are likely to be unwilling to even attempt to use the Time-Turners to do this.

“Hermione,’ said Harry suddenly, ‘what if we – we just run in there, and grab Pettigrew –’

‘No!’ said Hermione in a terrified whisper. ‘Don’t you understand? We’re breaking one of the most important wizarding laws! Nobody’s supposed to change time, nobody! You heard Dumbledore, if we’re seen –”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 21 (Hermione’s Secret)}}

In fact, through the Ministry, Voldemort’s enemies did have access to Time-Turners, since McGonagall got one for the Ministry for Hermione to use for her classes, but they never attempted to use them to change time to reverse any of his victories, even significant victories like his restoration to a proper body.

“Where did you get that hourglass thing?’

‘It’s called a Time-Turner,’ Hermione whispered, ‘and I got it from Professor McGonagall on our first day back. I’ve been using it all year to get to all my lessons. Professor McGonagall made me swear I wouldn’t tell anyone. She had to write all sorts of letters to the Ministry of Magic so I could have one. She had to tell them that I was a model student, and that I’d never, ever use it for anything except my studies … I’ve been turning it back so I could do hours over again, that’s how I’ve been doing several lessons at once, see?”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 21 (Hermione’s Secret)}}

Furthermore, using them is so risky, it may actually benefit him more to leave them in the hands of his enemies, because they could cause more damage to themselves than him.

“Exactly! You wouldn’t understand, you might even attack yourself! Don’t you see? Professor McGonagall told me what awful things have happened when wizards have meddled with time … loads of them ended up killing their past or future selves by mistake!”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 21 (Hermione's Secret)}}

He could also be assured that they would not be able to reverse any of his victories for longer than a few hours after them. If they attempted to travel too far back in time, Voldemort’s enemies would be certain to cause themselves serious harm, which would actively benefit him.

All attempts to travel back further than a few hours have resulted in catastrophic harm to the witch or wizard involved. It was not realised for many years why time travellers over great distances never survived their journeys. All such experiments have been abandoned since 1899, when Eloise Mintumble became trapped, for a period of five days, in the year 1402. Now we understand that her body had aged five centuries in its return to the present and, irreparably damaged, she died in St Mungo’s Hospital for Magical Maladies and Injuries shortly after we managed to retrieve her.
_{^{- Time-Turner (Wizarding World website)}}

It would only even be possible for them to change time to reverse one of his victories for around five hours without it being certain to cause serious problems for themselves.

As our investigations currently stand, the longest period that may be relived without the possibility of serious harm to the traveller or to time itself is around five hours. We have been able to encase single Hour-Reversal Charms, which are unstable and benefit from containment, in small, enchanted hour-glasses that may be worn around a witch or wizard’s neck and revolved according to the number of hours the user wishes to relive.
_{^{- Time-Turner (Wizarding World website)}}

Even going back in time one hour can be potentially dangerous, which is why Time-Turners are heavily regulated by the Ministry of Magic.

While not as potentially dangerous as skipping five centuries, the re-use of a single hour can still have dramatic consequences and the Ministry of Magic seeks the strictest guarantees if it permits the use of these rare and powerful objects.
_{^{- Time-Turner (Wizarding World website)}}

Since his enemies are likely to be unwilling to risk using them, if they did they would only have a window of a few hours to change anything, and attempting to do so is reasonably likely to cause them more harm than good, it would be a waste of his time and effort for Voldemort to steal all the Time-Turners.