This is just my opinion:
1) Because to secure the job career
If you are a fresh graduate a master or PHD degree not necessary secure a job.... nowadays, not much job require a master or phd with 0 working experience.
For the case of those having working experience, Master or Phd also isn't what employer is looking... it is more of the experience and project you handled before

2) Salary factor
A master degree or PHD will have a higher paid certainly... but 3 years and 5 years to get them... it is almost equivalent to the increment of start working after bachelor degree

3) Satisfaction
YES, I think this is the main reason, to realize own dreams

4) Migrate to other country
Again, experience play a bigger part... unless one migrate directly after getting his mater or PHD... but in order to migrate one needs to land a job first... right?

5) or it is own dream to teach people or passion in endless learning curve.
This is some what similar to 3) where it is one's passion to teach or to learn. Satisfaction gain from fulfilling one's passion