The expensive part of a storefront is not the UI front-end. And of thars all you care about, we have quite a few front ends that can do that job.
Why would anyone use slop alternatives though? I don't think people wouldn't be creating non-slop ones.
I think the larger question is how do those marketplaces get the rights to distribute any games... this is something all Steam rivals struggle immensely with, and it's not a coding problem at all.