I make it once every week or two, so they don’t have much time to go stale. The stuff on the shelf at the supermarket seems to have a much longer time in between being roasted and ground, and eventually finding its way into a shopping trolly. The bigger problem with ground spice mixtures though, is that they go stale at different rates. If the ground cumin seeds you have lose a little of their punch, it’s fine because you can just put more in. With spice mixtures, you can start out with something that’s well balanced, but the lighter flavors will start to fade, leaving behind the more potent ones (like clove or cardamom, in the case of garam masala). So when you go to use it, you taste the curry and it doesn’t have enough spice in it, so you add a little more and all of a sudden it has the over-powering biter after taste of too much cardamom, without much else.

The other benefit is that it’s obviously just going to taste better, or at least more the way you prefer it, if you make it yourself. Maybe I’m a weirdo though, because I don’t buy any pre-ground spices aside from chili (which keeps really well), and turmeric (which - in powder form - is used more for color than flavor).