No that would be a terrible idea. The problems with that would be that it would penalize the ones who were really good and reward the ones who were not and you would get a lower quality of black belt overall.
In school the really talented kids get to go into special "Gifted" programs and/or skip grades.
There would be less incentive for the mediocre to raise their standards because they know they will get their black belt anyway.
In school the ones who flunk are held back a year (or are supposed to be, anyway).
There have been some really dumb people who have graduated from high school.
Mostly due to artificial and bureaucratic machinations which link federal funding to factory-like production figures for students and, occasionally, "fudging" tests scores in order to meet academic standards requirements for students who are great athletes but rather poor students.
In schools there are a set curriculum and students start at the same time and learn exactly the same thing at exactly the same time and they are required by law to attend.
By the time I got to High School, I had a great deal of flexibility in what classes I could take. Sure I had to take a "math" course but it could be general math or some more advanced course. I actually took Latin one semester in HS.
In a martial arts school students join at different times in the teaching cycle (one student might learn a particular technique at yellow belt and another might not learn it until blue belt because that is what is being taught at the time). Some students train more often than others and some leave for a time and come back later so having the time for gaining black belt for everyone is just not practical. The two types of schools can't work the same way.
Actually, I'm aware of some schools that insist that beginners all sign up to start at the same time. They enroll beginner students for classes that start on a given date and run through a given date. It's usually considered kind of a "taster track" sort of thing but the concept is easy enough to extend. Further, it's pretty darn common to have a "advanced student" session in many schools, following the general class.
A school that guarantees a black belt in a set amount of time shows a major sign of a M'cdojo/M'cdojang and that's not a good look.
Standard "public school" model which PhotonGuy is talking about isn't supposed to guarantee graduation either. That they, apparently, sometimes do, was one of your complaints right? They're not supposed to and they do. Does that make those people who graduated in such a way holders of a McDiploma? Maybe.
I'm not saying I'd like martial arts instruction and testing set up the way "public school" is. In fact, I'd probably hate it. I'm just saying that it's a valid method of instruction and testing and seems to produce results.
What we're all used to in more-or-less standard martial arts venues ain't the only way to skin the cat.
Peace favor your sword,
Kirk