What are they doing when they run around the yard, or chase rabbits over 5000 acres? That's not mental stimulation or physical exercise? The reason I am building kennels is because one of the beagles is running more than she can eat and losing too much weight. I suppose you think I should kennel them and then exercise them on a lead every day while giving them math quizzes?
My dogs are well cared for. We have too many of them, but saved a bunch of them from the pound.
train them. Use them for hunting. compete in obedience. hunt competitions. field work. there are a million things you can do with dogs. And no, dogs actually DON'T exercise as much as people think when they are left alone to roam. They spend most of their time sleeping, like wild animals.
When left alone, dogs get into trouble. They find ways to amuse themselves, often in ways that your neighbor's won't appreciate.