It's an historical artifact from a time when people lived and worked on the nobility's land as literal serfs. The UK is just dreadful at progress, especially when it doesn't serve the elite. The whole concept of a 99 year lease was actually a major victory for commoners / compromise with the nobility and landlord class. While that compromise is no longer part of living memory (1920s) it's certainly still viewed as the furthest acceptable limit by many hereditary and institutional landlords.