I have seen protein suggestions based on all kind of factors: body weight, lean body weight or lean body weight modified by muscle mass.
The only one of those you can discretely measure is body weight. Body fat measurements are not really accurate and as a result you'd get an inaccurate lean body weight number. Same goes for determining the muscle mass.
Then you have different sized people, people with longer or bigger muscles and people that have thinner or smaller ones.
How the protein metabolism works isn't completely understood, too.
And lastly not every protein source is as valuable as the other. What counts are the amino acids that make up the different types of protein. There are some high value and some lesser value proteins. But if you combine two lesser value protein sources they can get high value, just because the proportions of needed amino acids evened each other out.
So as you can see it would be really complex to put all those factors, which have high uncertainties, into a formula to calculate a specific result. It would be absolutely inaccurate anyway, it is far easier to give you a discrete number.
In fact the 2g/kg are the maximum that can effectively metabolized, everything above it isn't going to waste, but most certainly isn't used for muscle build up, but converted to energy.
So instead of recommending you something that might be insufficient, Starting Strength just goes with the maximum as their simple estimation.