Because it takes a long time to calculate it manually
Basically, if a primary's xy coordinate changes, its Y target must change along with it, otherwise there is a non linear relationship between that primary and the white point. For example, D65 white in the rec709 space is comprised of target Y's of red 21.2%, green 71.5%, blue 7.2%. But those percentages only apply if those red green and blue are at their designated xy coordinates, otherwise D65 will not be produced. This is why different colour spaces have different Y targets. So if we change their xy coordinates, then their Y targets must change along with them as per the formula (taken from this article).
To simplify it, if a primary's saturation is too low, it's luminance (Y) must go up, in order to produce D65. If its saturation is too high, it's luminance must come down, in order to produce D65.
If a primary's saturation is too high, and you don't lower its Y, then the result is a non linear relationship between that colour and the white balance. What we ultimately want is a 1:1 relationship between the primaries and the white point they produce when mixed together.
I don't believe Chromapure or Calman offer this calculation, so it would be a real boon to have it in HCFR