Nowadays every big race has to have an announcer to keep things going smoothly. No, an announcer at a race is not necessary. Then again, I have been to many small weddings that didn't need entertainment. Some weddings want to have little party gifts for the guests. The centerpieces are raffled off to give some of the guests something to go home with.
Races have their T-shirts.
Now let's talk logistics. A wedding will often be held in two places, one for the ceremony, one for the reception. Guests drive to both places and stay there. Races send their guests running all over the city. I never heard a wedding planner having to hire police to close down the roads all around the reception hall.