Because you lose a lot of flavor out of a pot. It boils and steams away. That's the whole idea behind sous vide, it all stays concentrated in the food. And that's why you have to be very careful about using things like garlic or fish sauce, the flavors are super concentrated. Everything tastes more like what it tastes like! So rather than fish sauce tasting just umami, it actually tasted like fish. I think also keeping things just before a simmer might be helpful.