Garam masala, of course, is a blend of warming spices. However, many of those spices are listed individually in recipes for curry pastes and powders that also call for garam masala.
Since most garam masala mixes wouldn't specify ratios even if they do list the ingredients, doesn't that essentially mean you're randomly altering in unknown ways the ratios of key spices in the curry recipe?
When using garam masala within another spice blend rather than on its own, why not just list each desired ingredient of the main recipe in its ideal amount so everyone knows from which starting point they're tweaking and perfecting the recipe for their own tastes?
Best Answer
Recipe complexity is certainly a factor (as the other answers mention), but there's possibly a bigger reason: masalas often contain a very large number of spices that most western home cooks don't frequently use. While some people (generally more adventurous cooks) are willing to purchase a dozen or two spices and make good use of them, that's a bit much to expect of most home cooks. And if you don't use the spices much, you'd end up spending quite a bit on spices, and not even use them all before they go stale. A lot more people will be willing to make a recipe if they can just buy a jar of premade garam masala and be done with it.