The real purpose of pursuing a PhD can go beyond academic advancement and can vary significantly depending on an individual's goals, interests, and values. PhD programs are designed to sharpen critical thinking, analytical abilities, and problem-solving skills. These skills are highly transferable and are valued in a wide range of fields, from business to public policy to entrepreneurship.

Speaking strictly pragmatically, there are several reasons why somebody wants to pursue a PhD:

1. Some jobs are significantly better paid when the employee has a PhD degree.

2. Some academic circles are only open for persons with similar level of education, specifically PhD.

3. Consider somebody who achieved good result in some domain and wants to make it known for mankind. He or she tries to write an article in a scientific magazine, attend to conferences etc and quickly finds the only way to manage it is to pursue a PhD.

4. Somebody was born in a highly educated family, and seems obligated to hold the education level of family. Take the same situation under different point of view. Consider two brothers in such a family, one with PhD degree and another without it. The one with PhD will probably have some advantages of his degree, and this may lead another to want PhD degree as well.

To be fair, there are also pragmatic disadvantages of having PhD:

1. Consider a country with slowing economy. Some company has to free one of two employees, one having PhD degree (and thus is better paid), another has no degree. If their results in job are comparable, it is more probable the one with PhD will be freed.

Some people like science and PhD is the way to do some good things for the future generations. In real world most scientists like the ants bring some new knowledge that looks like a tiny speck and put it on the pile of Science, after that one of them develops something big. All scientists that you mentioned above would never do anything good if they would not have that information provided by others (little working ants).