Pessimism/optimism are mostly to signal social support for things. If you act with irrational optimism towards someone's ideas you are showing that you support the person. Similarly if you don't act with irrational optimism many will take it as evidence that you don't support them, which can cause them to become hostile against you.

This means that acting with irrational optimism is often the rational choice when you consider social factors, and that is what managers are looking for when the job ad says it wants an optimistic person.

However that kind of irrational social based optimism isn't what entrepreneurs means when they talk about optimism. Their kind of optimism is that you need to see yourself as better than others so you wont conclude that since most others fail you too will fail, which is kind of the opposite kind of optimism managers are looking for in their reports.