Because they are different things... sorta.
add_action creates an event that hooks to some specific action, which could occur multiple times and probably will. An "action event" is something like "WordPress is displaying the post content", or "the plugins have just loaded" or whatever. It happens every time the page is displayed, or at least, a lot.
register_activation_hook creates an event that only happens one time, when your plugin is first activated. This actually uses a hidden action which is "this plugin X just got activated", but that's beside the point.
Although both of these function calls happen every time the page is loaded, the actual code executed by your function is deferred until later.
See, anything in the "main body" of your plugin code runs *every single time the page is loaded*. add_action and register_activation_hook both defer the execution of some function until a given event occurs. That's what they do, that's how actions work.
The reason you use register_activation_hook to schedule wp-cron events, is because the wp-cron event is persistent. It reschedules itself (if you use a cron event that's not "single"). So you only want to schedule it once, not have it add a new schedule every time the plugin is loaded (which is every time the page is loaded).
Note that for any normal plugin, you'll want to register_deactivation_hook too, which will remove the wp-cron event (via wp_clear_scheduled_hook, most likely).
You might want to read up on actions in general: