Those people often run their sound through some cheap speaker system; so it doesn't matter what sound hardware they have, it all sounds the same.
If you buy a set of Bose headphones and a set of Sony headphones, it is only after listening to the Bose headsets that you realize the Sony ones are rubbish. I do not even like Beats Audio headphones. As for speakers, if you install £100 sound hardware and plugin some £40 set of Logitech speakers, you are not going to hear much of a difference.
Onboard sound-cards have improved dramatically, audio processors (yes there is are sound cards with processors on them) are much cheaper and smaller than they used to be and are found on some onboard solutions. These processors are mostly beneficial for DJs, music producers or pod casters; they record, run DSP effects and digitally equalize audio in realtime without chopping and skipping.
High signal to noise ratio is another thing, but the average gamer is not going to complain about noisy, fuzzy audio input; a professional game in a tournament would more likely want to hear what the other team members are talking about without fuzzy noisy feedback and echos in the microphone channel throughput.
In summary, you do not need a discrete sound card to enjoy a good video game or movie; but you will need one for creative means, live performances or professional activities.
There are some audiophiles out there who just want to hear everything; the low frequencies and the intricate details in the hi bands, the stuff you can't hear in common speakers via common sound cards.