When someone does a Sound Card for the PC, he does not make it as good as he could. He keeps in mind that by making a newer product, he`ll make people to buy the newer one instead.
They simply build to earn money.
But in Amiga things are different. Amiga designers try to achieve their dreams just like Amiga users. If mp3 playing slows down the system they put an onboard mp3 decoder. And when they do it they do it the best. When they find a way to make the card better, they try it, and sometimes they are successful.