Certainly in the case of D, adoption and real-world usage was/is a goal. Perhaps less so for MenuetOS and SymbOS, but even 'pure' hobby projects tend to enjoy public interest. They don't just live on someone's hard-drive in a basement somewhere.

That D's licensing issues are now gone, certainly benefits D.

Even if adoption isn't the goal, making it FOSS might still make good sense. Suppose the goals are novelty and technical achievement. More interest means more support in developing their novelty OS, no?