Because there are those who would want to steal the project for themselves. Giving free access to a stranger is a good way to get your project copied or stolen. It is not about being unfun, it is about being intelligent and careful with something the developer likely spent a long time planning and making by themself.
Beyond that, you never told me what the developer would gain from it. Why would he give give free access to his source code when it is still in active development? What can you offer him?
Viewing post in Another Space Simulator (A.S.S.) comments
if it was open source people could find bugs earlier, optimize aspects, and considering this is a solo dev that would help a lot
i don't think the idea people would steal it is really an issue at all, not only is the game already free, but people don't(or usually don't) use the strange html5 port of the game on some unknown website. and he can always ask the websites to stop using his IP, and also it be unlikely that a website or anyone in general would steal the game considering the type of people who play it, it wouldn't be popular, meaning they wouldn't see it worth it to steal. i don't see an issue with coping aspects in a game like this. its a space sim, it isn't really original itself. it is an attempt to get an accurate representation of a space ship. it would be nice to let others see how part of the code work. ig the whole source code would be a bad idea but some of the systems would be cool.