People already covered a few reasons deer hunters wear camo (only wanting one set of hunting clothes, camo does work on deer to an extent, etc). I'll add a couple of other reasons.

Many hunting seasons overlap, so someone might head out early on a Saturday morning to their hunting area to hunt for deer then spend the heat of the day hunting birds. Or maybe they have a license for both bear and deer, and they'll take what they see.

Second, camo works on humans. Non-hunters absolutely love to mess with hunters. I have several stories of waking up hours before dawn, driving to a place I can hunt, sitting in a tree stand in the freezing cold waiting for dawn, and then having a random person spot me from the road, then hike up to me to tell me I'm not allowed to be there cause Mr X doesnt let people hunt on his property (despite the fact that I have a signed permission note from Mr X). Or someone who doesn't think hunting should be allowed spotting me and then just letting their dogs off leash through the woods to flush away any deer, despite the fact that if I had a dog with me while hunting, it would be illegal because of how stressing it is for deer to be chased by dogs. Or the DNR officer doing their job by checking to make sure I have permission, the proper licenses, weaponry, square inches of pure blaze orange, etc, and next thing you know, you've lost 1 of the 2 days you'll be able to hunt that season. You only have a few prime hours per day, and having a person show up during that time will keep deer away for the rest of that window.

It's far easier to just hide from people.

Edit: and another thing I just remembered to add, camo lets other people know you are likely a hunter. If you are just walking the woods with shorts, a t-shirt, and a weapon, you will likely freak some people out.