Cus the time turner don't reslly work like that. Time keeps going forward despite you going backwards. Things you did while time traveling were already set to happen. Plus depending on how far back you go you would Age up the further you back you go when you return. Though from what I've heard, Cursed Child does away with these rules.
That's the beauty of the bootstrap paradox. Cause and effect are gone. Voldemort can just think "i'll use the time turner one day after the battle of hogwarts" (assuming he can steal it) and another voldemort will just appear beside him to help him fight lol
Time travel is fictional and ig it'll always be fictional but I hate this type of time travel in media. It creates all sorts of paradoxes, breaks cause and effect, etc. alternative timelines are at least supported by quantum physics