Because all the data used to decide is both subjective and qualitative in nature, that's why.
How does she look? How does she act? Are her legs tight? Are her ears perked?
Is she cleaning out the feed tub? Is she better than before the Preakness?
Better than before the Oaks? How did she look at her half mile breeze?
Listen, all of that stuff is important - but it's only half of the puzzle. The other half is based in exercise physiology. The objective, quantitative stuff is missing from this picture in all stables.
In the simplest of terms, how much blood (fuel) does it take for her to complete a 2 mile gallop, or a half mile breeze? How quickly does her heart get back down to 80bpm after that fast work? Compare her data during this weeks workouts to before/after her other races and get some meaningful numbers.
V200, described in this blog elswhere, is the velocity traveled when the horse's heart rate hits 200bpm. For those of Rachel's caliber, that number is around 30mph. This gives you a number over time that can show you when she is peaking, peaked, or ready to bounce - wouldn't you like to have that information?