A key indirect risk of a shutdown is the delay of vital data releases on labor and inflation. This forces investors and the Fed to operate in an information vacuum, increasing uncertainty and the potential to overreact to anecdotal signals, creating outsized market effects.
Prolonged government shutdowns don't end due to broad GDP impact. Instead, a specific, high-visibility failure of public services, like an air traffic control stoppage, creates the necessary political urgency for a resolution by making the economic risk tangible and immediate.