Because people confuse Microsoft marketing fluff with reality?
"Microsoft Edge is built from the ground up to improve productivity, to be more secure, and to correctly, quickly and reliably render Web pages. While Microsoft Edge is the default browser for Windows 10 and is the best fit for most users, some enterprise customers have line-of-business applications built specifically for older Web technologies, which require Internet Explorer 11." [0]
"We designed Microsoft Edge from the ground up to prioritize power efficiency and deliver more battery life" [1]
"Microsoft Edge is designed from the ground up to provide a modern, interoperable, and secure browsing experience"[2]
[0] https://blogs.microsoft.com/firehose/2016/05/19/improvements...
[1] https://blogs.windows.com/windowsexperience/2016/06/20/more-...
[2] https://blogs.windows.com/msedgedev/2016/06/07/edge-enterpri...
Marketing: it is all fun and games until you validate the claims.
New isn't always better. It's not as if no one on the Chrome/Firefox/IE teams are working on the architecture of the existing browsers. If you had the resources to build a new browser from scratch, you probably would not end up with something better than the existing browsers.
You don't just quickly write a browser from scratch in this day and age. And if you did, it would be so much better than Edge or the other contemporary browsers, because you could start out with a much better architecture...