That's not as universal as your statement seems to suggest. Most places I've seen TIG, the time shown is a pretty good approximation of how long it takes people to make the progress required for the next rank.
well... mileage may vary.
I suspect it can vary from school to school based on pedagogy, and methodology... But I must say that my opinion, [ based on observation of my students and having been a longtime (serial) student ], that a lot of places have a very long tig because:
1. students (some, not all) are lazy and don't pursue excellence (karate after school daycare, or doing it as chore for mom or dad)
2. the curricula is either inefficient, or not accelerated for those who are highly self-disciplined (ie those who are spending an hour or even three hours, afterclass, reviewing what they just learned at home) and apply themselves.(they have an intense daily practice)
and are well enough ahead of the curve, or well versed enough to promote if tested on the spot, yet are still 1 year out due to "tig reqs".
contrast this with the guys or kids that barely show up 1-2 times a week for class, and that's 100% of what they do. (and this is normative for many schools)
The tigs are for hobbyist level practitioners, and not for those who hve embraced it as a holistic / integral way of life... or are hardcore devotees who are on fire and are inhaling it as fast as you can throw it at them.
I admire the old Kodokan performance meritocratic dan promotions. Ie... if you can repeatedly execute your technique " under pressure " then you've earned that regardless of time spent under an Organization and paying dues. 3 ippons in a single tournament.
bam! first dan, or second, or third.
jmtc