I'm not saying $15 Million is small, but if in the future you or one of your family suffered serious disease, do you think $15 Million are enough?
Cancer patients pay $16 billion out-of-pocket annually for cancer treatment.
If you have cancer or other diseases that are considered incurable, even if you have billions of dollars, it won't save you, spending millions of dollars just to keep you alive for a few more years. Therefore, depending on each person's financial ability, there will be different decisions.
15 million dollars is not a small amount of money, mate. The US population is 400 million, how many people can have that amount of money, let alone other third countries? That amount of money will be enough for you to have financial freedom and be able to retire anytime you want.