Dan's blog is rendered as static, works without javascript and still lets him write complex client-side components when he calls for it. And if one day he decides to add a page that renders content dynamically, he can do so in the same patterns and framework that he's already using.
The goal is developer-choice. No need to pick static or dynamic holistically, have access to both when you need them. No need to pick between hydrating the entire website vs. rendering exclusively on the server. No need to pick between writing client-side amenable code or server-only code.
How many platforms have a "marketing" website for / and a platform website for /dashboard? No need to split them, just use a framework that accommodates both seamlessly. It's more powerful, even though it does come with a learning curve.
Components (be it Astro, Svelte, React, etc.) have a lovely API for sharing code across web pages. I used Hugo before and hit the limits of shortcodes pretty quickly. I can't comment on Jekyll though.
Furthermore, if the need for some dynamically rendered pages or client-side interactivity comes up, it is very easy to integrate this later on. I can build static, server-rendered and client-rendered pages (and everything in between) with the same set of tools, which means less mental load when I develop.
React might sound like a weird choice to you but its composition model including server and static stuff is quite powerful. See here for a slightly contrived but representative example: https://overreacted.io/impossible-components/#a-sortable-lis...
That's it. It's not a clever way to arrive at the conclusion that the server is useful, but they got there in the end.
All of this makes very little sense, but every couple years a guru will shout new commandments from his social media mountaintop and reshape the landscape. It's more of a religious ritual at this point, really.
On a more serious note, it's also because the market is saturated with developers who specialize on React, and this is what's easy and makes sense to reuse their knowledge and code (vs say, setting up a caching reverse proxy and dealing with invalidation).
With LLMs to help wrestle the boilerplate, I've found I can whip up a fast static site using advanced ergonomic abstractions that make the whole process a joy! In the past, wrestling the Node and NPM ecosystem was a a complete nightmare. Now it's, a dream — with the occasional storm cloud.