There's a _very_ big difference in quality between the iPhone earbuds and high quality earbuds/headphones, and a relatively small difference in quality between different DACs. And outboard DACs come with a lot of tradeoffs: extra hardware, battery life (or another battery to charge), cost, and so on. So there are many people who care about sound quality and invested in high quality earbuds/headphones, but rationally didn't spring for an outboard DAC. You get 98% of the quality without it.