It reminds me greatly my making of conlangs (artificial languages). While I find it creative, it takes vehement amounts of time to just create a simple draft and an arduous work to make satisfactory material. And all I'd get is just two or three people calling it cool and showing just a small interest. And I always know I'll get bored with that language in few days and never make as much as to translate simple texts.

And yet every now and then I get an amazing idea and can't stop myself from "wasting" hours, planning and writing about some conlang. And I end up being unsatisfied.

I don't think it is about Sunk Cost. It's more about a form of addiction toward creative works. Some kind of vicious cycle, where brain engages in activity, that just makes you want more to do that activity. The more you work on it, the more you want to do it, until reaching saturation, when you just can't look at it anymore.