1. We have not had much time to evolve away from such a mentality, so it's still very much bred into us.

2. One could say that gaining wealth is actually playing a strategy designed to preserve more than just your immediate offspring. The wealthy have less risk of seeing their line disappear in future generations.

Sadly, many studies have shown that a surprisingly high percentage of children are not the biological children of their fathers. IIRC, around 5%. And it's well known that many high-profile/successful men have many affairs.

This is fits in with evolutionary pressures.