I agree with you guys! There's a couple of problems though;
My skills are limited to using Adobe AIR at the moment, while I do know a fair share of C#/Unity it's not enough to create an application out of it. I'd be open if someone wants to attempt creating a version like that, but currently the Adobe Animate SWF files make it extremely easy to create the elements in the collections. I haven't found a file format Unity can work with that will have the same features.
The second is costs. It's fairly easy and quick to create new features for Kenney Studio NXT in its current form, creating a collection takes about an hour or two. The donations to my game assets plus the sales of Kenney Studio are on the low side and I'm having a bit of trouble to keep managing everything. That's why I try to go for quick releases to boost sales, instead of creating an open environment.
Hope you guys understand, I'm all open for ideas and thoughts though!