It is on chrome notebooks. And every search on google.com prompts you to install chrome for 'a better user experience'. Soon, just by plain nagging, it ends up becoming default

Sure it isn't. Had to set up a Windows 10 laptop recently, and the amount of hoops I had to jump through, just to wrestle the default browser from Edge...!

The default browser on Windows sucks and when you get online and go to Google you get an ad to download Chrome. That sorta makes it the default.

Google has deals with many OEM vendors for Windows laptops and such. Whether it's the platform default is meaningless in this context.