Probably because they're building their own engine from scratch. Many of the popular browsers these days are built on Chromium or Webkit. The only "big" alternative these days is Gecko, which is what Firefox uses.

This matters because Chromium based browsers make up the vast majority of usage and Google has been using Chromium to drive web standards in the direction they think they should go.

Wikipedia has an overview, but doesn't really cover Chromium's market capture. https://en.m.wikipedia.org/wiki/Comparison_of_browser_engines