There's a number of reasons, but one of the biggest is that it's impractical for an individual or small group to create a web engine that is capable enough that users will want to use it. This means the only options are to either fork an existing browser (as most Chromium forks are doing) or build a browser around an embeddable engine (as WebKit-based browsers do, and hopefully one day Servo-based browsers will).
For many interested in creating a browser, a new engine is one of the primary reasons for doing so, and so forking or embedding being the only option means that many who would've created a new browser don't, because from their perspective there's no point in a Chrome/Firefox/Safari clone with a slightly different coat of paint.
WebKit at least partially addresses the clone issue, making it easy for developers to write entirely new UI code using their toolkit of choice, but comes with the caveat of not receiving much attention on non-Apple platforms, which is a problem with browser security being so important.
Also, saying 'Chromium fork' is mostly untrue since these browsers almost always change stuff that's required (ie. changing the appearance, logos, chrome:// uri, and maybe solving some platform-specific bugs) then continue to pull changes from upstream Chromium with little intent to iterate on the core components of the browser or implement new standards. Brave is the one exception with IPFS support but that's it - not even MSFT did something like enabling PlayReady in chromium, it's straight widevine.
For many interested in creating a browser, a new engine is one of the primary reasons for doing so, and so forking or embedding being the only option means that many who would've created a new browser don't, because from their perspective there's no point in a Chrome/Firefox/Safari clone with a slightly different coat of paint.
WebKit at least partially addresses the clone issue, making it easy for developers to write entirely new UI code using their toolkit of choice, but comes with the caveat of not receiving much attention on non-Apple platforms, which is a problem with browser security being so important.