Why is garam masala in many curry paste/powder recipes?
It sounds like you're assuming that recipes are scientific creations carefully engineered to achieve a precise result. But most "recipes" are an attempt to relay a rather imprecise series of steps based on available ingredients, familiarity, superstition, and habit in a way that is approximately reproducible by someone else.

Even within the bounds of attempted precision, most cooks execute the instructions slightly differently because their experiences of what works and what doesn't work is different, their skills vary, and because they interpret the instructions differently due to language ambiguity.

Imagine you're a cook who grew up cooking Indian food based on how your mom or dad cooked. When she says "we're having brinjals" or "we're making channa masala", that creates a picture in your mind of how the dish is supposed to taste. When you see mom grab a handful of garam masala, it's because she knows roughly what it will taste like, and tweaks the seasoning a bit based on her perception that dish X benefits from more cloves and dish Y benefits from more coriander seeds. Now you decide to write down the recipe so that you can reproduce it in case you forget the details. You are not going to send the garam masala off to the lab to analyze what ratio each spice is present in there. Garam masala is just the "spicy blend" and "biriyani masala" is just the blend of sweet spices you use as the foundation for a seasoned rice dish, and chaat masala is the starting point for things you put on fruit or snacks. You don't need a deeper level of detail, because that's what you've always used.

Then you go to work in a different region, and you find out that the typical garam masala sold by the spice vendors is completely different; this one has a ton of cloves and a lot less cumin. You react one of two ways: "Oh, I should add more of X to compensate", or "Well, it's close enough."

In your mind, the function of the garam masala is to provide a starting foundation so you don't have to give as much thought. It has the base notes you expect in a wide range of dishes; when the dish benefits from some other spice, you add a little of that.

This is how most of the world cooks. We don't necessarily start with a recipe and get a dish; we start with a dish and then someone asks us for a recipe, and we furiously attempt to write down the details of what we do the next time we make it, probably changing the dish in the process in our attempt to achieve some form of measurability and apparent precision. Why 1/4 tsp exactly? Well, that was the unit of measure I had handy when I decided I needed "a little of this."

And this is also why it doesn't matter so much. You assume there's some ideal; in practice, there is no ideal, there's just what you're used to. Basic technique often makes a bigger difference in the quality of the dish than variations in the exact ratio of the spices. Toast your fenugreek, but don't burn it. Sautee onions until they're tender. Simmer, but don't overcook. These kinds of steps matter more than whether your garam masala has a 1 part cumin or 2 parts cumin.

There's another factor, too. One of my Indian cookbooks actually does contain a canonical ratio of garam masala and a few other spice blends based on what the author expects; you can use hers, or just buy pre-blended ones, and you'll still get pretty good results. But if you are a cookbook author and every recipe contains a list of 3 ingredients plus a list of 15 spices, 12 of which are present in your usual garam masala, you'll get the opposite complaint that you're presenting: "Hey, this is too many ingredients for a weeknight meal. What is the author thinking? That we're all professional chefs or housewives with hours of free time?"

There are at least three very different segments in the world of recipe consumers. One is the person who attempts to follow the recipe with slavish precision, regardless of how complex, because they think that will give them the best result. Another is the person who will look at a recipe and say, "Huh, spicy tomato soup with shrimp. Oh, this combination sounds good. Except I hate tomatoes, so I'm going to use a white sauce as the base. And I have no shrimp, so I'll just use some clams I've got in the fridge. Oh, I don't like fenugreek very much, so I'll drop that and add some more cumin." The third is the mass consumer of recipes who doesn't actually like cooking very much, and they just want to add a little variety to their weeknight dinner routine and don't care about authenticity, precision, or detail, and they just want something a little new to them that they can put together in spite of poor knife skills, inability to understand complex directions, and so on. Nearly every recipe is a set of compromises designed to address the concerns of at least two of those constituencies.