Why doesn't Space.SE use icons yet?
A simple explanation is because the site is still in Beta. All Beta SE sites (except the sponsored one, like QuantumComputing.SE and ArtificialIntelligence.SE) will have the generic beta theme, including favicon with an SE-bubble with either letter or symbol inside it.
The more complex detail is... since Design-Independent Graduation, sites can have graduation without site-design. You can see the list of graduated SE sites without site-design (including favicon) on this answer.
FWIW, all of you are free to propose any site icons for this community (e.g. Lifehacks.SE had a short discussion on that), but until this site has graduated, and until SE Design Team continue the site-design project, this discussion might be moot.