Because there is some probability of it happening. There is a trend of research and logical implications that are required to be fulfilled before more progress can be made in some specific direction.
For example, suppose two men each have a garden. For the last year a severe drought has plagued the lands and little food exists. One day though it must rain because nature dictates it to be so(if it didn't rain life on this planet would cease to be life and we could not even discuss these things, and so there is always a change of a downpour in a drought). Both these men's gardens will be replenished with water and they will both bear the fruits of the rain. The other men who gave up will not have anything to show.
When some discoveries are created by some men, it was done on the backs of others... and then new men jump on their backs. It's how things are done. Because literally millions upon millions of humans work on the same things constantly there will be simultaneous events. Two men might read the same paper independently and come up with the same idea that thousands of others didn't... because of a whole host of reasons... because the two men happen to think alike or had similar experiences.
It's like asking why Pi is 3.141592... It is because it is. It had to be something and so something was chosen. The universe works in the way it does because that is the way it has worked.
I have rediscovered countless mathematical theorems in my journey through mathematics by simply generalization some result... only to then find out later that it was something already discovered by someone else. This is how it is for many. They are a dollar short and a day late, but this happens to everyone. In fact, many people discover things and it was never published and someone else took the "credit".
As science becomes more specialized and all the low hanging fruit have been picked, the "gems" require more devoted pickers who are willing to climb the tree of knowledge. The good news is that the tree is always bearing fruit and for those interested in it they learn to look, they can get it... of course, there is always someone who is trying to get their first.
E.g., it's basic mathematics and probability. You will have some people who beat others and by probability there will have to be some likelihood of both beating each other(meaning it occurs together).
E.g., suppose the probability of someone discovering some important thing in any given moment is p.
This means you have a p chance and so do I. If we run this experiment a bunch of times most of the time One of us will discover it first... but there will be some occurrences where we will discover the same thing on the same day. One can make this more explicit by studying it mathematically(e.g., a Poisson process). It is the same as when two light bulbs would go out at the same time, or two fail safes would go out... it is unlikely but it happens.
Given that most of the people who spend their entire lives trying to discover something and they want to be famous, they will go for the largest most ripe fruit on the tree... hence you have millions of people trying to prove the RH... So every day people are working on it and it will just so happen that there is a chance two people will figure it out on the same day.
But lets get real: These things actually don't happen instantly. A discovery is not a light bulb that goes off but many years of research. It's more like a race where you have the top 10 finishers... but the top 3 are really trying to win. If they see one guy pushing it then they will try harder and try to beat him.
What this means is that if one guy is about to publish a paper(which takes time) and someone else hears about it they will work hard to publish theirs first. These things actually don't happen simultaneously both are given credit because they both essentially came in first. There is no photo finish in these things because it's not so much about personal glorification as it is about scientific progress.
So it's more of an illusion. Two people don't actually stub their toe on the exact same couch model at the exact same time on the exact same planet wearing the exact same type of clothes at the exact same.... (each exact makes the likelihood far less)...
It just seems like it happens "exactly" because all the details are missing. There is really no such thing as "perfection". Even a baseball player who hits a home run doesn't hit the ball perfectly. He just hit's it perfectly enough for everyone to cheer.
So, you can essentially blame it on your ignorance, and as you learn what really is going on there will be other ignorant people who will ask the same question and have to learn, we all ask the questions and that is how we learn. That is, we all are constantly discovering things. You can bet that there were probably 10 people on this planet that was thinking of the exact same type of thing(but not exactly of course) that you were when you posted it... At least probabilistically.
Why it really happens? Well, you will have to ask God that...