Using spices doesn't mean making spicy food, especially if you're using spicy to mean containing capsaicin. They are mostly used to enhance the main flavour of the dish, they don't need to be overpowering.

And sure it adds umami, but if that's all we wanted we could just use the fish sauce it's based on. The spices add additional flavour that add more than just a generic umami flavour profile. Garlic is umami too, but that's not its entire flavour.