Its not default.

My mother doesn't know how to turn off the phone, toggle airplane mode/wifi or use the app switcher... She refuses to learn because its not worth her time. I have family, friends and such that similarly refuse to learn basic things, much less go seek out an ad blocker and learn how to use it for an interface (the web browser) they think they rarely use.

Also, on mobile, it is kinda tricky because you can install an adblock browser, but the app selection is kinda iffy, and it won't replace the unfiltered system webview.