Because software monocultures are bad. The vast majority of browsers are Chromium based. Since Google de-facto decides what gets in Chromium, sooner or later the downstream forks are forced to adopt their changes. Manifest V3 is a great example of this. You can only backport for so long, especially when upstream is being adversarial to your changes. We need an unaffiliated engine that corrects the mistakes we made with KHTML/Webkit.