Why do we wear any kind of style, really? Not a lot is actually functionally designed throughout.
No, but clothing tends to evolve in a natural fashion, even over centuries. Shoulder pads have traditionally been the domain of tailored clothing, which this doesn't resemble. Further, since the 18th century, the overarching trend has been clearly and dramatically toward increasingly casual wear, with less structure and padding. That's one thing I enjoy about the TOS costumes -- they got it right that in the future, people would go to work in what could pass for sleepwear.
It's not impossible that we could one day decide our Under Armour needs shoulder padding -- just look at the Golden Girls -- but it looks odd to my 21st century eyes.