I’m honored to have an 11-marathon veteran use or plans!
At the risk of sounding snarky, why a 14 or 21-day taper? Why not a 16, 19, or 24 day taper? Surely, there is an optimal period that is not constrained to an integer calendar week.
Of course, we use full weeks because most of us are not professional runners, who can make their training blocks be 8 days or 5 days or 12 days instead of a convenient 7 days.
In reality, there really is an optimal taper period, but it is unique to each athlete. It might actually be 17 days for you and 12 days for someone else.
But, as a plan author and coach, we write plans for the masses, and it’s a commercial decision to use full week increments.
What you are really asking is: accepting the constraint and limited options of either a 2 or 3-weeks taper, why do you use 2 weeks?
We feel that the empirical evidence leans towards a shorter taper over a longer taper, and so we skew the “typical” runner to 2 weeks. Remember, it also depends on how aggressive the taper is. A 3 week taper will be less aggressive than a 2-week taper, and our two week taper is aggressive.