Browsh does!

Not having control of the core codebase, and branching/tracking based on 1 (declared) legacy feature could lead to huge amounts of work and issue in the future.
Manifest V2 spec is defined, manifest V3 spec is defined... They can be developed against.
JS-whatever-spec is defined, CSS-whatever-spec is defined, HTML-whatever-spec is defined... They have industry standard approved specs (even if they can be vague in areas). They can be developed against.
They have defined spec documents that can be developed against.

Firefox has control and experience of how they implement those specs.
Chrome forks do not have control of how those specs are implemented.
So if chrome changes how things are implemented, forks might not be able to "backport" for manifest V2 compatibility, and might find themselves implementing more and more of the core browser functionality. Browsers are NOT easy to develop for the modern fuckery of the web.
Firefox hopefully does have that knowledge and ability to include V2 manifest backwards compatibility in future development without impacting further spec implementations.... It seems like Google is depreciating V2 to combat ad-blockers (ads being their major funding revenue)

There are already very slight differences how Firefox and Chrome interpret all these specs. I've noticed a few sites & plugins that just work better (or just work) in Chrome. Which is why I still have (unfortunately) an install of Chrome.

A port of a browser is relatively minimal effort. Typically, the changes are largely cosmetic, and occasionally skin deep.

There's a reason none of the ports of Chrome caught the recent snafu with Google having its own special addon that fucks your privacy.

Developing a browser, Firefox or Chrome, takes a huge amount of effort, and are on a similar scale to both Windows and Linux. It's a lot. There are a lot of places to hide things. Taking all of that, and making V2 continue to work... well it'll be alright to start with. It's probably a flag somewhere currently. But in 2 years time? 5 years time? It will take a lot to keep V2 working, let alone back porting V3 features that people may actually want.

Just use Firefox instead.

Why can’t they just fork the last usable version of chromium and go from there as an independent fork? Is it just that no one wants to?

Creating or even just maintaining a web browser is an insurmountable amount of work. With constantly changing and new specs coming out all the time, it's an unwinnable amount of work. Not to mention, browsers and the Internet in general is so complex it's like web browsers are an operating system themselves.

A web browser is likely the most complex software on your PC outside of the operating system itself.

To clarify for anyone curious about the drama, while it was blown out of proportion, it was absolutly vaild.

1. there was a light nsfw furry easter egg, removed once found. Considering the browser was originally a side project by a young guy (teen/early 20?) it's not really surprising or a big deal. Once the browser gained a sudden boost in users and it was found, the image was removed (once the guy got back from vacation? hospital?, there was a month or two gap)

2. this one was a larger problem for sure, and again removed. If I reacll right, he was apparently hosting a website for a friend about supporting the end of a certain procedure done to baby males at birth. There were some graphic images, its not technically CP anymore than the infomus Nirvana cover, but still...not okay.

To make matters worse, the link the site was somewhere browsers home or about page, making it pretty easy for anyone to find.

It's all old news now. Personally I didn't really care, but some people might.