Btw the same goes for when every junior leaves in 6 months. Or whatever demographic group.
I've been in software professionally for over 20 years and in that time I have only left 1 employer before 5+ years. I left after 8 months and it was because the company and org and team culture were a horrible, broken, dysfunctional mess, and the people who were capable of changing it didn't want to admit why they had the problems they had in the first place.
You've had a somewhat traditional career there by HN standards. Most of Silicon Valley was promo-hopping ever 1-2 years for the whole middle period of your career (somewhat curtailed now by the job market). It was really popular to FAANG -> startup for 6 months -> FAANG-1-level-up for a good chunk of the 2010s
The work was essentially web scraping complex systems, which is not a case of "you don't need someone with my experience to do this" and more like I just didn't want to do it because it did not improve my current skills at all.
It is not like in the interview they told me: you will be doing web scraping for months on end. Or that I even thought to ask.
I do think that startups are pretty rough when it comes hiring. We don't pay BigTech salaries, we expect engineers to wear a lot of hats, and we don't offer the kind of management chain isolation a big company does. It takes a particular kind of engineer to enjoy/thrive in that environment, and a lot of engineers transitioning from big companies don't realise the extent of that until they've struggled with their first startup role.
And that is just when you can talk to the engineers directly, it gets worse when you need to escalate on the management chain.
Startups are easier because most of the time you usually only need one rubberstamp (and sometimes none!) to do something.
It gets much worse though once the startup scales up and becomes massive. Then it tends to be worse than big corps because there at least they have processes.