Why are some wealthy people so frugal?

Because wealth and frugality are related. You can become wealthy in very few general ways:

You can inherit;

You can invent something that people want to buy;

You can be amongst the most hard working, capable and talented at something people will pay for;

You can rent seek (use connections to take from others that which you do not deserve), or;
You can save and invest (live below your means).

As you can see, most of that list is not available to most people. One item, #5, is generally
available (this may be a bit of a USA truth, I cannot speak to other countries).

Bonus, if you live your life according to #5, then your children have a chance for #1.

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