Garam masala is a catch-all term for an Indian spice blend. It has no fixed recipe but is likely to contain a combination of cloves, cardamom, cinnamon, bay, black pepper, star anise, dried chillies, coriander, cumin and maybe more or less. Blends vary according to family tradition and region. Spices are then dried out and possibly roasted, before being ground to dust.
The downside is that if you find garam masala hard to find you may well find the components equally tricky to source. In the UK we are spoiled by a wealth of Indian grocers who stock these ingredients in reasonably-priced quantities. These spices are also readily available in any supermarket due to our historic connection to India.
For a starting point that aims towards the curries that you have specified, I would combine 20g cloves, 50g cardamom seeds, 100g cinnamon sticks, 5 bay leaves, 75g black peppercorns, 100g coriander seeds and 100g cumin seeds. Some toast the seeds in a dry pan but to avoid scorching and to squeeze out all the moisture I use the Heston Blumenthal technique: dry-roast the spices in a very low oven (100C / 212F) for an hour and leave to cool. Blitz to powder in a coffee or spice grinder. I would recommend storing for up to 6 months in an airtight jar to preserve its punch.
Remember this is only a starting point and can be completely customised according to how you like your curries.
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.
Best Answer
Garam masala powder is a spice blend, other than some anti-caking agents it's almost pure spices. Pastes are spices mixed with oil and maybe water and other ingredients like garlic, chili, pepper, tamarind, and others so they aren't an exact equivalent. Taking garam masala paste in particular if you look at patak's product the spices are only 23% of the total weight, the rest is water, oil, garlic powder and tamarind paste. You would need to add 4 tbsps of paste to get the same amount of spice as 1tbsp of garam masala powder, plus you will be getting extra flavors like garlic and tamarind in the mix.