Also most people simply cannot write good documentation especially for complex topics.
If you are an electrician you also label your terminals and add plans to avoid troubles for other electricians and your future self. People who don't do that are handymen, but not electricians.
Many developers are handymen, not engineers.
When you write your thoughts down, you can then read them back to yourself. Then you can see all the flaws and gaps in your thinking. Then you can edit your document to refine your thinking. And if you work on a team, you can then share this best-representation of your thoughts with your colleagues and get their feedback. If you're not doing this then you're almost certainly not creating optimal designs.
Maybe you think this is all overkill, and maybe it is some of the time, but I can't tell you how much stupid code I've read that was written by smart people who weren't thinking clearly.
Using documentation to clarify concepts as I go is certainly aomething that happens, but it is not the reason i do it.
You can get any development project done without documentation; I don't think that's up for debate. The question is, can you make changes to this project, improve on it, explain it to another team during an acquisition or due diligence: 5+ years down the line? And if you can, will that new development timeline be accelerated if you had documentation and diagrams to work from?
What happens when a new CEO comes in, hires a new Engineering Manager, and 85% of the devs quit in protest? Will a new team be as effective as possible without that documentation?
In my experience the above scenarios are quite common in start-up and "scale-up" size companies. Where there are a lot of things in flux and nobody has a plan 5 years out. Then, lo and behold, it's 5 years later and everyone is stuck picking up the slack instead of developing new/improved products.
And you're right, it can be a case of misalignment of goals and vision; not everyone cares about the long term. But if your company isn't thinking of problems further out than 6 months then you might have a different problem to address...