Because, as pointed in the page, Servo is being developed as a(n embeddable) Rendering Engine, not as a full blown end user Browser.
Its alternatives are not Chrome, Safari or Firefox, but Webkit, Blink and Gecko
There’s an example GUI called Servoshell, but it is more of a testing ground and example on how to embed the engine in an app than a serious alternative to anything currently in the market.
Already this kind of work is difficult and daunting. Adding to it a full GUI would make it completely impossible for the current size and financial backing Servo has.
Big words aside it just means that Servo wants to be only one of the parts that compose a real browser: the one that takes HTML, Javascript, WASM and translates them into the things you see on your monitor. All the user facing functionality are left to the devs of the app that embed it.