Because they make money on them, and many people who mod Mustangs really have no idea why they do they things they do. They just think Aftermarket stuff must be better.
The issue is Ford's design introduces a lot of bind. Then you have the mindset that rubber bushings are sloppy, so lets stiffen them up with urethane, and you just make the bind worse and uncontrolled, which can lead to snap oversteer.
The herd mentality is also strong. Hop on Facebook and make a post stating you want to keep your quad shocks on yoru car and see what happens. You'll get 100 posts laughing at you telling you to throw them in the trash and buy aftermarket arms with the stiffest bushings you can find. Nobody wants to actually listen about the correct way to modify a suspension. (which unfortunately is expensive).
The best Fox Mustang rear suspensions do away with the Quadra-link setup and convert to a 3-link, or 5-link setup. With a panhard bar setup you can even try a Poor-mans 3-link setup. Hop on youtube or google and you'll see a ton of info on these setups. EricthecarGuy did an awesome series on installing an entire Max Motorsports suspension setup on his fairlane. I've got most of it on my car other than the torque arm (for now).
But...what are your actual goals with the car? You don't need a torque arm to cruise to local car shows. In fact, the NVH increase from one may be a discouraging factor.
It's also no secret that Fox Mustangs became famous by going in a straight line. A lot of the aftermarket reflects that as does the interwebz echo chamber.
It's also no secret that crappy movies have made popular the idea that one suspension setup for the track means it is SuperReallyAwesomeSauce on the street.