You may indeed be correct about 37% not being in the Workforce, but not being in the Workforce is not the same as the U6 Number.

There is a specific calculation or set of calculations that is used to compute the U6 Number and just subtracting 63% from 100% is not part of the Calculation.

Calculating the Unemployment rate by simply Subtracting 63% from 100% and saying that what is left in this case 37% is the Unemployment Rate is not Logical.

What about people who are Retired, they are not part of the Workforce but should they really be considered to be unemployed and then there are the people who don't want to work and receive Welfare should they be considered to be unemployed?

It all depends on what you consider Unemployment to be and or your definition of Unemployment.

If your definition of Unemployment is, anyone who doesn't work, regardless of reason that is one thing, but if willingness to work or ability to work or being retired is taken into consideration then that is a totally different thing.