Consider this situation from the perspective of a biological person. A biological person has resource limitations in terms of effort, concentration and resources. When you're already worth say 10bn each, your primary limitation is time and effort, which doesn't scale. Do you really want to squander your personal time and effort on increasing your net worth by 5%? Not likely. You have the next horizon to conquer and you have plenty of cash.

Let Zuckerberg have his measly $750m, which means F all to either of you and get on with your next project.

This is entirely a pragmatic and reasonable decision on the part of the whatsapp founders.