Transferable Financial Knowledge

We are taught to diversity our investments to reduce risk as if our life depended on it. Although spreading your money between different stocks is still not diversification because stocks correlate so much with each other.

We are also taught to have a rainy day fund, plan for plan B, have alternatives.

Why in the world then are we taught that we should just stick to one job? 40 (probably more) hours a week?

When we put all our eggs in a single job, who's the real benefactor?