I'll give you one instead of grand parent: to protect the company.
If you fire your CEO over an inappropriate relationship with an employee, the company may come out looking strong and principled.
If you fire your CEO over fraud, your company will look dubious at best and you can see your stock tumble.
An example of specifics provided for this EXACT reason: Priceline Group (more Bookings Holdings) being fired over a relationship in 2016. The stock didn't budge.
NB: To be clear, Booking Holdings actually IS really principled about holding execs accountable to the code of conduct. It's something that I always appreciated a great deal when I worked there.