Why Can We Only Send One Squad Out?

I have 8 squad members, but I can only send 4 out?
Why can't I send out two separate squads? I have 4 level 40 squad members and 4 level 27 squad members, simply because my first squad is pretty well balanced and there's no value in switching them out.

I KNOOOOOW!
It sucks super bad.
You get 8 people in your squadrons, and really basically just use 3-4 of them and let the others rot, especially since once you start unlocking more mastery points on someone there's no reason not to use them. Like, even if the mastery points aren't in what you'd like, the one that gets upgraded is always random so having more points nearly always = better.