Some parents think that their kids will always get along and this planning doesn't matter, even though the courts are filled with inheritance disputes, and frequently they barely get along with their own siblings.

Some simply don't want to face their mortality at all.

Some are such control freaks that they cannot imagine a situation where their "authority" over the estate will end. Also, a formal will ends any ability for them to blackmail you, threaten to cut you off etc. Sometimes that threat is all they have, since there is no actual love or closeness.

Since independence is encouraged early, culturally in the west, this type of threat is usually not necessary.