A subset do, for sure. But some of us are just playing around.
For apps, it matters less but you still have to think about how easy it is to modify. That's why we care about open source, right?
A good pitch? Why does that even matter? 97% of these projects are for fun/as a learning exercise. If someone is interested those things, then they can check it out. If they aren't, then I guess the author misses out on a few GitHub stars. shrug
Sometimes it is done just to learn, but it seems a bit silly to do it in public as it were.
I mean how likely is it that "Hashtable/JS interpreter/JVM implemented in C" is going to be a big hit on HN? Not often. But since Go, JavaScript, and other insert new hotness language here are popular things somehow it's more interesting to retread old ideas with them. That's not the sign of a young master demonstrating his skill, it is the sign of an immature field that hasn't figured out how to distinguish what is new and important from what isn't.
It's unfortunate in particular for the people who do do these projects for fun or learning. These people have their work hijacked for a showoff culture's mutual backpatting.