Because she is head of HR and he is CEO; two positions in an organisation that simultaneously require a level of trust between the two individuals but also a degree of distance and separation, with an element of Chinese walls being deployed, at least in theory, because (again in theory) the former is (to some extent) supposed to provide a level of protection to the workforce from the latter.
Whilst HR’s ultimate function in an organisation is to protect and defer to senior management, ostensibly it’s there to protect the weak from the strong. It’s hard to imagine a more inappropriate relationship between two senior executives in a large corporation.