why is it impossible to block all calls from a specifik country

That is not a setting that is available on the iPhone. You can speak to your cellular carrier if you wish and see if they have something on their end that can block calls in that manner.

You also have the option of providing feedback to Apple to let them know that you would like to see a setting like that included in the future. Your asking for that does not guarantee that it will happen, but that is the only way Apple gets feedback from users. Feedback - iPhone - Apple