I am among those people, 2 programming jobs, 12 hours a day typing non stop, for 15 years so far. I am probably super lucky but have never had an issue on my wrists and have used every single shitty keyboard you can think of.
I love an ergonomic keyboard, but it costs 50 dollars maybe? This is not that, this is something else.
It's also mechanical, it's got open source and reflashable firmware, it's hand-assembled by well-paid workers in Taiwan, it's covered with a 2-year warranty. (The landing page has a bunch of other "nice features" listed.)
I'm not sure what kinds of keyboards you've used, but there are good reasons to buy ones like these.
This specific one might be a little on the "premium" cost side, sure, but I think it's entirely reasonable that it is and that people would still buy it. The list of good things about it is long enough that I'd consider it if I were still in the market for a keyboard.
$300 or whatever it is is a lot, but in the USA it's a rounding error compared to the cost of healthcare for a problem like RSI, and in Europe it's a rounding error for a business compared to the cost of an employee unable to work, or suing because they were not given appropriate (i.e. ergonomic) tools. A DIY version (solder it yourself) is half the cost, but insurance/employer won't buy that.
For others, it's a tool they use for many hours every day. I spent €10 on my kitchen knives, and my friend who is a professional chef is appalled, but I wouldn't want to use his €200 laptop every day.
They're probably only going to sell a few thousand, so there isn't the economy of scale that makes something like a Microsoft Sculpt $100. Finally, it's made from individual components (key switches), so should be easily repairable if a single key should fail, but also took a lot more time to assemble. It ought to last many years.
What's the point of judging how people spend money on the internet? Are you saying you are above reproach in your spending in every facet of life? You don't buy anything because you just enjoy it, or because it strikes your personal fancy? If so, good for you, but don't say things like "I don't mean any offense" and follow that up immediately with "I find it insane that..." The second statement is clearly intended to convey some sort of offense.
My two cents (as someone that doesn't have a fancy keyboard).
People will gladly spend hundreds of dollars on "fancy plates" (aka china) that sit on display in shelves. Thousands or tens of thousands of dollars could be paid for paint on canvas, just because the paint arrangement is unique, or the person that did the painting is well known. People pay 5 dollars for a cup of coffee at starbucks.
I think it's just important to realize that spending isn't always a strictly rational process. That said, there are definitely significant pieces of engineering that go into a keyboard. Here, the keyboard is fully programmable and you can flash the firmware. For some people's workflows, this might be very important. No keyboard in your price range will allow that. This keyboard is equipped with a low-power ARM cortex, which means it can handle additional logic a typical keyboard can't. You can use "layers" and swap between them at a hardware instead of a software level. This means you can take your keybindings with you as opposed to needing to change the OS settings when you switch computers while using the same keyboard. If you have actual wrist issues due to an injury, frankly I would pay even more money than this to ensure that I can type pain-free or that the symptoms don't deteriorate. As someone that's worked in hardware, I can say first-hand that the plastic injection molds are extremely expensive for products not sold at a massive scale.
I could go on. I'm not a "keyboard collector" or anything, but I do have a keyboard that cost me over 100 dollars, and having used it for eight years now, I consider the price entirely reasonable given the utility its afforded me, and the longevity. Not to mention that I can replace keycaps if needed, I can replace the cord, I can reprogram it as my needs change, etc.
I can understand if English isn't your first language, that's not a problem. Tone-wise, I think if you genuinely are curious about something, just make that clear. Otherwise, the question comes off as a rhetorical one, where you're "asking" in the sense of "why anyone would be so stupid as to spend this much money is beyond me."
I love an ergonomic keyboard, but it costs 50 dollars maybe? This is not that, this is something else.