Not just that, they also rebuild parts that have fallen over or broken, even if it wasn't broken in our lifetimes. If they can clearly identify where a certain part belongs, they will put it back there.

So it's not just preservation, there is actual reconstruction going on too.

Adding on to other answers, they do restore them somewhat. A lot of Greek temples and other structures look like ruins but are actually already partly restored, columns re-erected, parts replaced. The intent is to restore it enough to give an impression of what it would have looked like, without straight up guessing or causing damage to the parts that are there.

We COULD restore them with reasonable guesses, but they'll still only be guesses. Better to restore to a point we know, and let the mind do the work.