I’ve often reimplemented things at work that exist elsewhere. If I could just copy & paste whole solutions from GitHub and change the branding/naming slightly, I could make curl in an afternoon.

I can only think of hobby projects, like writing yet another emulator, expression parser or media processor in a new language I'm trying to master.

In a professional setting, you would always diligently explore libraries and only implement your own if there is no suitable alternative.

Only when the existing free solutions are licensed with something like GPL. Now I can just say, write me a C webserver library similar to mongoose and I get the functionality without the license burden.