And I'm saying that that pattern doesn't exist, at least no more for scientists than for any other field. (If you broaden your field to other than politics, then think about all the other people who have switched careers. President Obama was a law professor earlier in life. Sting was a schoolteacher.)

I gave a list of famous scientists who did not move well beyond the scientific area where they made their name. I'll add more: Cyrus Levinthal, Dorothy Hodgkin, Hermann Weyl, Edsger Dijkstra, Maria Mitchell, Abraham Maslow, John Wheeler. The list goes on and on. Again, consult a list of Nobel Prize winners and see how it's no more common for "famous scientists in a particular field" to do what you say they do than for any other profession.

I posit that you have sampling error, in that you've mostly only heard of scientists who are known in their field and are also known for work outside their field.