Why should it be? Not everything has to be FOSS. There’s no real benefit from having the thousandst archive website be FOSS, every fifth grader can code something like this.

Sure, the site’s developer would partially benefit due to security updates, but this would be offset by the fact anyone can copy their site and scam people.

Imo it is more important whether the site can work with no js and no cookies than it being officially foss.

If you can throw js out of the window - the amount of bad stuff that can happen is drastically reduced. And what you can already see in the html source of the page is literally what you get.

And yes, there is such amount of websites (incl foss ones) and making them is so easy that maybe the constructive benefit to the humanity of opensourcing a particular one is minimal.
At least unless it is literally a thing that threatens the privacy of billions of people. But those are not going foss or js-optional any time soon.