Microsoft Edge User Agent string

So Why does it have Mozilla, Chrome and Safari, and not just Edge in it?

Webpage designers are simply lazy, and they attempt to block visitors from using specific browsers, because they want to use the easier framework which is only supported by certain browsers.

It also is a deliberate design decision to limit the usefulness of user-agent checks. This decision likely based on the fact (from the perspective of Spartan/Edge Developers), is that in the past, websites would look specifically for IE and notify users to use a different browser.

Since Edge/Spartan was Microsoft's first attempt to modernize the Trident engine, and Edge/Spartan's purpose was to match the feature set of (Blink, AppleWebKit, and Gecko) at the time, the user agent simply advertising itself as being everything is one way to prevent Webpage designers from easily targetting Edge the same way they targeted IE.

Many websites that you visit today will not display well in IE because they are not coded properly and usually display a page like they would in an old browser, even though Internet Explorer 11 supports many of the new web standards. Some pages will display incorrectly in IE while working perfectly fine on other browsers such as Chrome or Firefox.

There's also a problem with jerky scrolling that doesn't go away for some websites even after you turn off smooth scrolling, and there are cases in which important elements will not display because a webpage identifies your browser as IE.

Microsoft's new Spartan engine masks itself as Chrome

Chrome and Firefox also does something similar:

Mozilla/5.0 (Windows NT X.Y; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2657.0 Safari/537.36,gzip(gfe),gzip(gfe)"

It is worth pointing out that IE11's user agent is similar:

Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko

User-agent string changes