Because of how the game is structured... trying to flank yourself is a practice of suicide since moving closer to the enemy gives you a chance of drawing more enemies.
There are a few exceptions to this but generally speaking... you do NOT want to experience 9 aliens attacking you at once. I've only fended something like that off without casualties twice.
Can't you just use the classic X-Com method of putting a rookie in front to die first and then having the veterans blast them from the rear?
You can, to a point. Remember, though, that in classic X-Com you had 14 soldiers per mission, many of which could be rookies with nothing but electroflares (so they didn't panic and kill your vets). Here, you've got 6 soldiers, which gives you fewer slots for expendable scouts. Also, remember that in classic X-Com
everybody had the 'Squadsight' perk, whereas now you need to move your vets within firing range after the rookie spots the aliens. (Or wait patiently for the aliens to come to you...)
If you're going to put something out front to draw fire, consider a SHIV (more health, can't panic, can shoot half-decently) instead of a rookie. You're still better off not attracting the attention of 9 enemy forces at once, though.
...in fact, one of the best tricks for avoiding alien swarms is to abuse battle scanners. The aliens only get their scatter move when
they see
you, not the other way around. Battle scanners let you locate the aliens without them locating you, at which point the game kinda glitches out and gives you visibility of their HP bars even after the scanner shuts off. You can then trigger their scatter move whenever you want... say, by tossing a grenade over a wall, or free-aiming a rocket at them, or even landing a Squadsight shot if you're quick. Pre-scatter, the aliens tend to be bunched up and out of cover, which means a preemptive attack is ridiculously powerful.