Being hacked is only one of many security concerns. The authors of software don't always value the users wishes over their own, open source at least keeps them

Being hacked is only one of many security concerns. The authors of software don't always value the users wishes over their own, open source at least keeps them honest about what they're doing. A closed source browser (that I'm sure has some "telemetrics") who's main goal is to analyze browsing patterns is a privacy nightmare.