CEO is a political position, not a productive role. The job of the Chief Executive is to be a high level influencer with lenders, high level clients, and other business partners. For AIs to fill this role, they would need to be established as more influential than their human peers.

While its certainly possible (and arguably the desired end result of Microsoft/Google/et al) to replace a C-level with an AI, the end result would be a machine that serves the interests of the operators (presumably Microsoft/Google/whomever) rather than the business for which it is providing the service.