With code, there's a certain attitude of "I need this, so I'm going to make it, but I have no intention of ever productizing it so why not share it?" It's a massive "give a penny, take a penny" jar that enriches the entire software ecosystem.
The analogous situation for graphics design would be something like textures & Photoshop filters. Or icon packs. Or fonts. Which you can find lots of for free (sometimes even with useful licensing attached).
But who the heck is going to come do free graphics work for my new Flask extension when they don't even use Flask, or understand what it is, or have any motivation beyond "hey come do free work for me"?
On the other hand, you have cases like the current mpld3 logo update. The project's author asked for help with the logo on Twitter, and got 6+ solid responses back. People are glad to help if they have the skills to do so and if they're part of the project's community. Design vs. development is not relevant, only relevance is relevant.