This post turned out to be a bit of a rant about what drives me to model my own designs most of the time. In short, it isn't required, but I highly recommend it.

I'd say that most people who own 3D printers have little to no skill in modelling and are happy printing whatever they can download online. Maybe they hit a point where they want more, but until then learning modelling isn't a useful skill for them.

Personally, I'm a designer at the end of the day. 3D modelling is a crucial tool in taking my ideas and bringing them to life in a way that can be passed to a manufacturing process and made into a physical object. 3D printing just happens to be the manufacturing tool I use most often for personal projects because it is what I have the easiest access to. If I had a machine shop, I'd use that too. When working on high volume products I'll design for injection molding, die casting, sheet metal, compression molding, etc.

I'm not against utilizing models people have already put online that solve the problem I want, that is just efficient use of resources. But I agree, most models out there are very poor quality so I pretty rarely use downloaded models. Heck, I just re-modelled Gridfinity bins because I couldn't find a parametrically adjustable model for SolidWorks that I was happy with (on that note, the dimensional documentation for Gridfinity is straight garbage and I'm still not sure I have it right) and those are some of the most widely available models out there.

I also absolutely despise STL and other non-parametric file formats for sharing designs. They are terrible, inefficient formats that make files very hard to edit. Most people don't export them in high enough resolution resulting in horrible looking faceted models. The community needs to fully accept STEP as the file format of choice now that any slicer worth using can import them properly.