Why did they not just negotiate? Why did they not just have a battle? Why go to the trouble of building such thick walls which may or may not ever be attacked?
These are tough questions. The answers have to do with cultural ways of thinking.
The ancient Greeks did not build tall walls around their homes. The ancient Romans only built wooden walls around forts on the Northern edges of the empire. Saxons built wooden walls around settlements.
Normans, however, liked to lay sieges and defend against them. It was how they thought about land that made them do this.
It was how they thought about the value of someone’s word that made them reluctant to negotiate for land.
For more info on how the Normans thought differently than the empires which had conquered before them, you can find several good books at your local library.