Why was the final Invasion mission hidden behind all the other missions?

I don't like this mode and was playing it only to finish missions.

I have already achieved an S rank at least once while playing another mission.

Why can't the game look at my highest score in the mode and see that I've already achieved an S rank? Why wasn't I just given this mission from the start so I could have worked on it simultaneously with other missions? Why is this mission just a boring score goal instead of anything that actually alters how the mode is played? Why does it require a 5 man premade when it doesn't have any weird requirements in champ select that would breed rage between anonymous players?

The mission requirements are stupid enough already and I already have played this mode way longer than I wanted to.