Sure, but many people want to use Strava for more than one purpose.

a) Analysis and tracking of your own personal goals. (Some of the tools are better than the stuff available on the device itself.)

b) Sharing and socialising some other activities.

You can be careful and only allow certain activities to be public but you'll make mistakes and eventually many people will just think "whatever, I'll just default to public and remember to hide the ones I don't want to be public" and then it's even easier to make mistakes.

Defaulting to "opt-in" is all well and good until a human makes a mistake.