Because they are living WikiTree members. The member can choose between 4 private levels: Private, Private with public biographie, Private with public family tree, Private with public biographie and family tree.
The profile manager can also choose a privacy level for his profiles. Profiles of people born over 150 years ago or who died over 100 years ago must be Open.
Living people who are not WikiTree members are "unlisted".
See here:
https://www.wikitree.com/wiki/Help:Privacy
