I think the reason is best described by Factorio. Yes, with a lot of spaghetti belts and manual crafting, you can make late-stage products. But to do it well, in a way that transcends merely being able to make a handful of items, you need a huge underlying economy. You end up having to re-arrange your whole factory, or you need to clear new land, which means you need to spend some time on diversions like improved mil…
A nice explanation for the "rate of progress" is the percentage of the population that is not involved in the production of food. Since the dawn of humanity up until "very recently" in the grand scheme of things, something like 95 to 98 percent of the population were directly involved in food production. There were the odd shamans, priests, tribal leaders, or what have you, but everyone else made themselves busy hunt…
Your problem also isn't just inventing better methods, you also have to rearrange society to take advantage of this new possibility.