No one spends hundreds of millions on audio engineering. Or at least, not at achieving the best audio experience. The company that spends the most on audio R&D

No one spends hundreds of millions on audio engineering. Or at least, not at achieving the best audio experience. The company that spends the most on audio R&D are probably Bose, and their whole goal is to produce speakers that look good and sound acceptable. The reason DIY audio gear matches really expensive factory built gear, is that the secret to good sounding audio is good sounding speakers, and good sounding speakers are heavy and big, and have intricate cones made out of expensive materials. Imagine how expensive it is to manufacture such a thing and have it marketed and shipped, and you'll realise they need large margins on something that's already expensive. Combine that with that the actual construction of speakers is very simple anyone could it do with a basic set of woodworking tools, not to mention that many people enjoy doing it as a hobby. Of course good amplifiers are also part of the story, but their development is a sort of by product of the computer industry, as better transistors has generally meant better amplifiers, with class D amplifiers basically being the state of the art right now.