Yes, people can change the default search engine at any time just like how Windows users could change the default Internet Explorer web browser at any time, yet the U.S. govt went after Microsoft and won.
The problem is, as studies have shown,
more than 95% of people keep the default settings. It's almost as if it requires too much effort for people to do/change things. Companies know this.
It's the same thing with tipping. When you're presented with pre-set default tip percentage options (e.g. 15%, 18%, 20%, 25%) along with an "other" option to set your own tip amount or "no tip," a lot of people choose one of the defaults because it's just easier.
You can extend this to businesses who opt you into everything because they know a lot of times customers won't bother to opt-out (e.g. sharing customer data with other companies).