Survival of the selfish

It’s difficult to believe that pure money motive, or lust for power & being feared drives someone to join realpolitik, mind you, & not referring to the drives (or circumstances) which make a politician successful, but those which make a man or a woman choose politics as a career, & later, take such career decisions as seeking a party ticket to contest, or changing their party, or forming a new party.

However, when we look at samples, rather than census, we often come to despair. Take, for instance the political themed movies. You surely get the hint at the latest you watched, the sagas of waste.
These are case analysis by the layman (non-political-scientist).
As a recurring theme, we see, that the most idealist vanish from the scene fastest. Either suffer career suicides, or if in the land on gun rule, actually cease to exist.
Those who act only for themselves, (& “themselves” in a very narrow sense), win. Thrive.
Why is that? Maybe the more abstract the ideas, weaker the conviction?
Or more precisely, the closer one’s political will to one’s on self-interest, the more energy & desperation employed.
Self-preservation is the essence of politics.