Thanks for the question, it's a good one!
While there's no doubt that integrated motherboard audio has improved over the years, one can't achieve good audio by simply putting a decent DAC on a motherboard.
A good audio design requires a good analog section. There are many contributors of noise on any motherboard, so designing a good analog section on the noisy motherboard is almost a defeating cause. On a powerful gaming computer, the noise from the powerful CPU is even worse. And this problem is magnified by onboard WIFI, Bluetooth et cetera.
The final nail in the coffin is the multiple GPU cards found on the most powerful computers, which makes the motherboard audio unredeemable.
A dedicated sound card can offer so much more, especially if it is the accumulation of many years of experience and expertise resulting in pristine audio design.
Our latest Sound Blaster X3, which has also been reviewed here on HEXUS, will attest to that