Because they don't have access to the required agents, tokens, etc. Because they have not thought of using a tool like the published one as a solution to whatever problem they're facing. Because it saves them the time going through the vibe coding phase, telling the agent that this lot that needs to be changed for the thing to work. Because publishing the results doesn't keep you or anyone else from not using them by using an agent to build something similar or just building it themselves.

If I planned on vibecoding a project, and during preparation I found a project that loosely fit my model, I may grab it and try to retrofit it to save on token consumption. If that had too many kinks, I'd probably start fresh, but it would be worth the initial attempt IMHO.