I think that may be though because you've been caught up in the Western view of a black belt ("complete ninja, ready to kill someone as easily as look at them, expert") rather than the Asian view ("well done, you've learnt the basics and are now competent enough to get on with the rest of your studies"). Remember in Korea, it's standard to get to 1st Poom in a year of training (and contrary to our view in the west, not all Taekwondo instructors in Korea are expert wisened old men that know Taekwondo perfectly).
The other side of it aside from expectations/meaning is is the more practical side. There are inevitable peaks and troughs in any athletic progression (and Taekwondo even as a martial art, not sport, is still a physical athletic ability and training programme). Without the regular training up for gradings and competitions and other events you don't experience the peaks. It's those peaks that push your ability, the focus period where you're nailing details to prepare for an event/grading.
So, I understand what you mean, but consider how good the guy who got his black belt in 3-5 years would be if he'd only graded 4 times in that period instead. Grading him "fast" has enabled him to improve considerably.
My view on this has now changed over the years, so I understand where you're coming from. I now want my black belts to be comparatively good compared to those produced by my peers, but I'm not caught up in they have to be perfect to get to 1st Dan. Now when I promote people to 4th/5th Dan that's a bit different... ;-)