why dont battery keep a charge on 2006 Ford Freestyle

battery checked ok charging system says ok but alternator reads doides pass but low reg,

Asked by for the 2006 Ford Freestyle
Voltage regulator output should be above 14.0 Volts at 2,000 RPM engine speed. If it is not, this is your area of attention.

Since the voltage regulator is internal to the alternator, you are likely going to need to replace the alternator.