These days, whitetails look up. Back in the old days, the treestand was a magic bullet, and it didn’t have to be very high to work.
It’s uncomfortable to the deer to look up. Its neck is more rigid than ours, and its eyes are situated to detect ground-based predators. But we’ve taught enough generations of whitetails that they need to keep an eye in the trees as well.
When deer get this wary, you only have a few options: Seek a new, less heavily hunted area or climb higher. Getting farther up, say 20 feet or more, re-establishes your aerial advantage. However, it also increases your risk. If you go to great heights while deer hunting, use solid climbing gear and stable stands, and always wear a safety belt.