Why should I write it myself? Sure, I could do that, but then every time I want to have that sort of functionality on multiple pages I would have to write the JavaScript code multiple times. I could then roll it into a library. Maybe make the library work with custom HTML attributes. And now I've just reinvented HTMX. So I might as well use HTMX instead of reinventing it.

Such a weird question. You could ask that about any library ever.