[unreal37]

Firefox, Chrome et al exist to serve ads. That's where they get their funding. That's who pulls the strings. Turning off cookies would kill that model for revenue.

[Hupriene]

Just to clarify and provide some supporting documentation: https://en.wikipedia.org/wiki/Mozilla_Foundation#Financing

Mozilla makes money by providing users to companies that make money from ads.

[ihaveajob]

You're spot on for Chrome, but I don't think you're familiar with Firefox and the Mozilla foundation.

[unreal37]

Mozilla Foundation reports $500 million per year in revenue. From search engines, primarily Google.

So, they're not turning on "private browsing" as the default.