I think the crux of the problem is: brilliant developers exist (and we tend to follow + see their work more often), but sub-mediocre developers also exist and are way more abundant. If you're looking at a pre-existing code base at work, it's more likely it was created and maintained by the latter rather than the former.

I follow great developers via social media/news and always tend to compare myself to them too much. That leaves me with the impression I'm a failure. But then I just have to go to a local tech meetup (in one of the biggest cities in the US no less) to be facing people with an appalling lack of knowledge, who like to discuss platitudes as if it's the second coming, and who really barely know what the hell they're doing and yet think they're the most clever people on earth. The feeling of wasted time is often an infuriating experience. Many times I've had to excuse myself during Q&A rounds after a presentation, given the outright ignorance displayed by people asking questions. Still, that's one experienced that serves to show me I'm probably doing something right.

I know I sound very negative and judgmental, so apologies. My point is, that's one thing I always recommend to people: go to local tech meetups. You can see how you "compare" there.