Because while we have conclusive data that states "the vaccine will keep you out of the hospital", we do not yet have the data that lets us conclusively say "you can't get and spread the virus if you're vaccinated".

Israel's data thus far looks promising (https://www.reuters.com/article/health-coronavirus-israel-va...), but they're being careful because they don't yet know for certain.

Again, "I don't know the answer" isn't the same as "there is no answer".