Because often it is the case that third party cables are of higher quality than stock cables, if you purchase from reputable vendors

Gamer judgement on "quality" is often misplaced and skewed towards cosmetic novelty though...

Often, but not always and whilst using third party cables may bring cosmetic appeal, many third parties provide quality cables.

Yes if you buy random cosmetic cables from random Amazon vendors you're running a heavy risk. However there are reputable 3rd party companies that provide both quality cables and cosmetic customization built for your particular power supply.

The same goes for any 3rd party products/accessories.

they're cheaper?

That sounds like someone buying a Lamborghini then trying to save money by fueling with regular gas.

But with gas the difference is at least somewhat legible to the layman. You see the octane rating and know you're getting something for that extra money. With cables they all look the same to the uninitiated, after all it connects doesn't it? So people are going to go for the cheaper option because they can't discern a reason not to.

The type of person who owns a 5090 must also be the type of person who knows about the 40 series and it's cable problems. Little timmy building his first PC wasn't camping outside of microcenter for a week just for a chance to spend $2000 on a video card

Yeah, the people that buy a Lamborghini are some of the "cheapest" people and will squabble over the smallest of things.