Ah, this would explain why when flying the Cessna 172 I was denied taxi clearance yesterday evening for my first mission flight out of KITW with clear, daytime in-sim skies. The real-world conditions at the time were most certainly IFR-only. Super confusing. This really needs to get fixed. Upvoted.
UPDATE: In my case, even when the airport is VFR, ATC still denies taxi, which is captured by this post. Workaround is to request IFR clearance.