Yeah I wondered that as well when I first started. At your stage of the game, you haven’t unlocked the ability to play old missions. You will very soon, though.
Edit - This is a good time to point out that replaying old missions is almost required if you want 100% unless you “cheat” by looking up mission objectives on the interweb. Extra objectives are hidden until you complete the mission once (or accidentally discover them during play) so unless you are very lucky you won’t stumble upon them all during your first play through of the map.