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. In some cases the registration area, start and finish are different locations. At most weddings the staff who make sure things go smoothly number about 20. The Market Square Day 10K, New Hampshire's largest 10K, had over 100 volunteers. Don't forget, at weddings the staff is being paid.