Because everyone has to learn some time, and what is obvious to one person isn't obvious to the next one. And anything which helps people realise something true is worth repeating from time to time.
Aside from the last part of that, I have some sympathy for people who make such comments. Often ideas are presented as "new" which are covered in any basic undergraduate textbook in the field; if someone working in the field isn't aware of these ideas, it simply means that they didn't do their homework.
There's a place for brilliant new ideas, but it generally comes after reading a book (or doing a google search) to check if a problem has already been analyzed to death.
I graduated from a university that is known for its CS department. I can still remember most of the concepts that were introduced. I can also imagine that if you work with a narrow slice of that curriculum for 10 years after graduation, you might forget or get rusty on a good number of unused algorithms or ways of thinking.
But I have also seen a lot of cases where different universities focus on different things in their curricula. For example, my university did not mention the phrase "functional programming", although that concept was well-known for over 15 years before my attendance. I was blissfully unaware of first-class functions. This is not new stuff, but several years after I graduated, I discovered it for the first time.
And imagine my surprise and glee when I discovered over six years after my graduation first-class classes, multimethods (or generic methods), and the like. Suddenly, languages like C++ and Java -- which a lot of universities force-feed their students -- seem very restricting. Yet all this is fairly basic, just different ways to think about and approach our problems.
The reality is that college can give you only so much, and you must accept that others coming from other colleges have been exposed to a different array of concepts and know about those more than you. You have to continue your career forward where the college left off.
And it sure helps when a place like Hacker News can occasionally bring up these various subjects. On the one hand, it exposes different people to new ways of thinking, and on the other hand, it might encourage other readers to join the field.
This stuff is old hat. But mentioning that and that you thought about it back in 1843 is a meaningless ego massage that, no, does not deserve sympathy. In a significant system, you cannot develop alone. Bring others up to your level, and learn from them, too.
I agree -- I did say "aside from the last part".
Perhaps the "this is old hat" people are just saying they only heard of it themselves yesterday or perhaps - as is very common - something is indeed obvious after you've heard it for the first time.