Because where it's at, it knows the area. It has known trails and tunnels and stays in these safe spaces. Move it out of it's area and it has no cover - no trails to run, no holes to dash down, making it very easy prey. If you're 2 inches tall, strongly scented, eaten by everything and a mile from a building, you're not making it to a building before getting snagged by something
I do NOT recommend moving and releasing most animals, mainly for this very reason, it's going to die and it's usually much better to just kill it yourself. But rats are really hard to get a humane brain shot on, OP is a vegan and ground-dwelling rodents are an important food source for just about everything. So some hawk, owl, fox, etc gets an easy meal, a chipmunk then gets to live another day and there's one less rat in the barn
Traps that kill, rats will see a dead rat in and learn to avoid them, 100%. But there's nothing inherently evil or scary about a live trap, so there's no reason to avoid that. But if you drown or kill a rat in a live trap, the pheromones of terror and adrenaline will cling to it, and other rats will avoid it