They try to make them as quiet as they can, but efficiency and cost are the deciding factors.
I’m not sure I entirely agree with this point. Clearly the materials needed to dampen the noise output exists. I’m thinking that none of the manufacturers will promote an extra quiet product that isn’t in their premium model. It is probably a so-called “value engineering” decision. Sure price competition has something to do with this decision, but marketing decisions, that is, the decisions by those who promote the product to a likely group of buyers at price point over-ride engineering for efficiency or quiet. In this case, it appears that the degree of “quiet” is a pawn in a bigger scheme. So the deciding factors becomes what the manufacturers think they can price certain features at. Certainly nothing new! I do agree that due to recent advances in bearing and magnet technologies used in modern fan motors are far more efficient.
Also I thought that a roughly 80% efficient gas furnace has been a standard for about the last 40 years or so? If not what was the standards from the ‘60s to the ‘80s?