If a house will be opening before the traditional call time (which sometimes happens with Stay and Scream), then there will only be volunteers who can show up early in the house. A similar situation can occur at the end of the night.
Then there are the unfortunate situations where a guest terrorizes a whole house, so most of the cast ends up waiting at the exit to point them out to security.
Basically, light runs happen. They’re not really Universal’s fault.