Well, for one they know that default browsers get more users than non-defaults. So being the only browser vendor without control over a OS is a huge disadvantag

Well, for one they know that default browsers get more users than non-defaults. So being the only browser vendor without control over a OS is a huge disadvantage. They also know that with Gecko and Trident/EdgeHTML having insignificant marketshare on mobile there's a greater chance a monoculture will ensue witch will cause issues in standards development and hinder other implementations.