Let's say you started taking the fictional Skribs Kwon Do twenty years ago. It's the best martial art ever, in my completely unbiased opinion. It took you 15 years to earn your black belt. You were the third black belt ever in the art, after me (the founder) and my first student. 15 long years, in which you had to learn everything under the sun, including groundfighting, take-downs, seventeen different clinches, joint locks, joint breaks, pressure points, a hundred striking combinations, and a dozen forms. Your black belt test had a fitness portion that would rival the NFL combine, three full days on techniques and concepts, and enough written essays on training, teaching, and self defense law to satisfy a triple major in bio-engineering, education, and criminal law.
Now, 20 years after you started, and 5 years after you go through all of this, I take a 5 year old student who has been training with me for 3 months, I give her a test that consists of 5 punches, 3 kicks, and breaking a piece of balsa wood, and she gets her black belt!
On the one hand, you still have all of the other stuff - the knowledge, the fitness, and the technique. That's what you care about, that's what others will care about when they see your technique.
On the other hand, if Gerry is looking for a school to take his daughter to, and he sees that you're a 1st degree black belt in Skribs Kwon Do, and then looks up a video on Skribs Kwon Do and sees a 5-year-old 1st degree who knows 8 techniques, he's going to wonder what you can teach her.
This is what the cheapening of the black belt means. (And yes, the story is super-exaggerated to highlight my point). It doesn't mean anything for you, until it does. It doesn't affect you, until the reputation catches up to you. Even worse, if this girl can get her black belt in 3 months, why did it take you 15 years?
Another way of looking at it is the opposite of power creep in TV and video games. There's a show I love (and actually started watching it because someone in my TKD class recommended it) called Supernatural. There are some monsters in the first few seasons that are incredibly difficult for the heroes to fight. They may spend an entire episode on one of these monsters, and it's something that has them scared witless the entire episode.
In the later seasons, there may be a dozen of these monsters, which are supercharged by some magical beast, and they're just cannon fodder to be mowed through like lawn gnomes. They're no longer scary, and there's no real impact.
It's kind of the same effect. If a black belt is really difficult to earn, and has very stringent guidelines on how to earn it, a black belt is something that generates a healthy fear and admiration. But if a black belt is basically a participation trophy, then the reaction is "meh". And even if you had to go through those stringent guidelines, people see it through the lens of "meh" because of what they've seen others have to go through to get their belt.