Some of the contradiction can be resolved by clarifying the terms you're using in your second sentence there.
"They". Is your first "they" who say that a black belt indicates a serious beginner the same as your second "they" who make the black belt so hard to get? (Hint - different people have different ideas about what a certain rank should represent.)
"1st degree black belt". What sort of black belt are we talking about? What art, awarded by what organization or instructor? Do you mean the local TKD kiddie care program, where a black belt might be awarded to a 13 year old who has trained 3 hours per week for 2 years (300 hours total)? Or do you mean a BJJ academy where a black belt might be awarded to a practitioner who has been training 8 hours per week for 10 years (4000 hours total)? I don't know too many people who claim that a typical BJJ black belt is just a beginner.
"Beginner", "Expert", "Master", "Hard to get". Depending on where you are in your training, those terms can indicate very different things. The first martial art I trained in for a significant length of time was Bujinkan Taijutsu (formerly branded as "ninjutsu"). This was back when the art was pretty new in the United States and there were only about a dozen or so black belts in the country. Most of them had only been training for a few years and they had learned from someone who had earned his rank based on a few, relatively short, visits to Japan.
At the time, I thought these guys were amazing fonts of martial knowledge. I was part of a small club in Baltimore that brought in these black belts to teach seminars and eventually I moved to a new city so that I could attend a dojo that had multiple black belt instructors. But from my current perspective as someone who has been training various arts for 42 years, just about every one of those black belts was a novice who barely understood any of what they were trying to teach. (And I am far from being any sort of "Master". Just an experienced martial artist who is reasonably competent in what I teach.)
So ... once you get past generalizations and realize that different people have different perspectives and that a particular gelt rank can mean very different things in different arts and schools, then it's really not so contradictory after all.