Building your own is not the same as buying, though. What would the cost for that same product be if it came all assembled for you, but the same quality? I'm betting it's pretty expensive.
It's probably also not that expensive to roast your own coffee beans, but it's an extra step that non-hobbyists don't want to deal with.
You can grab some nice drivers, unreasonably high-quality crossover components, and couple sheets of MDF to make yourself some floor-standers for a few hundred. Using any number of popular DIY speaker designs, you'll have something that would easily stand up to anything of a similar format in a blind A/B test.
Amplification is dirt cheap now; class-D amps are superlatively good even in the <$100 category. You can even add in DSP for a very reasonable price if you want to jerk off to glass-flat charts.