Why unlike Canada, UK, EU, Australia & NZ do US employees only get 10 days holiday, yet the rest get at least double or far more? ?

There is no required number of paid holidays in the US.  The AVERAGE is about 10 days of paid days off for new workers, but it varies by industry and experience.  For example, by the time I retired in the software industry, I had 6 weeks of paid leave per year.

Paid time off is just another employee benefit and, like all employee benefits, should be negotiable.  Some employees might prefer more time off and other employees might prefer a higher salary with less time off.  There's no justification for government enforcing a single one-size-fits-all approach.

Because if you think having some time for yourself is a wealth, then America has a piss poor economy.

Read Milton Freidman, University of Chicago Economics, explains it all. The corporation must maximize profits at the expense of the workers and the culture.

No one has thought it through to that degree. In a free market, the amount of vacation/holiday that an employee receives is entirely up to the parties involved, without government intervention. Most salaried employees get 2 weeks paid vacation (10 working days) plus around 8 paid holidays. If they want more time off than that and the employer agrees, then it is unpaid leave at that point. The US military gives 30 days paid leave, and the US government civil service has a variable leave system based on rank and seniority. 

P.S. It is hard to run a business with so many employees taking time off all the time. 

All part of being a socially backward nation.

Military giants, ethical pygmies 

In The USA, employers believe in working their employees to death.