Because in EIRR you win through holding territory, and being aggressive is detrimental to your call in timers...
thats one way to win, sure. The other is to simply slaughter your enemy. And one could say being aggressive is bad for callin timers for any faction/doctrine(lightning war? ami riflespam? and so on...)
However, you can easily just flow around the enemy spearhead and backcap..unless you play on something thats like 3 fuckin sectors wide...then obviously its far more difficult and holding ground is necessary, but thats a product of terrible map design, not poor balancing/game design. Been playing deserted village a lot the last few days...that map has a ton of sectors, its pretty nice.