Josh...I understand your point, but it does not take into account the value of time and labor. DIY is cost-effective from a materials standpoint, but the 20-30 hours it might take to assemble an amplifier or speakers from scratch is more effort/trouble than some people are willing to put up with (let alone the time it takes to become skilled enough to learn to actually build something).
Also, if I'm an audiophile and I want one pair of speakers, I'm not going to set up a carpenters' shop in my basement and buy all the tools just to make that one pair of speakers. Even if I wanted to change my speakers every 3 years, I don't know if I would make that investment.
You could apply the cost-effectiveness of DIY to a lot of things...DIY bicycles, beer/wine, furniture, etc. For many people, it is an excellent option. But for the majority of folks out there, particularly in the go-go USA, time and labor aren't on their side...and the option to pay someone else to do it is completely reasonable (and not getting "raked").