Exactly, why don't all these chromium-based browsers which came out against WEI don't fork Chromium to maintain a base version without this bullshit? And manifest V3 while they're at it.
It's likely a lot of work to maintain a fork of the Chromium/Blink engine with your own changes applied to it. I'm not sure how deeply the Web Integrity API is integrated into the code, but if it's anything more than a flag to disable it, it will likely be hard to keep integrating upstream changes timely while ensuring your fork still works.
Although Chromium/Blink is forked from WebKit, it's far from being WebKit these days.
But of course, Vivaldi could base their browser on WebKit or Gecko. Many of these "smaller" browsers tend to be based on Chromium though, likely because it's the most compatible (because of its marketshare).
And it's likely too much work for them to switch engines now.