Add to this that humans in the first moments of an ATCS game shy from going out in the yard. For two reasons: one, the downstairs entrance to their base has 2 turrets by default and they feel it's reasonably protected (and it is, somewhat, from dretches, for a while); so they leave that way alone. Two, the yard is large, aliens can be anywhere, and beyond it lies a heavily protected alien entrance.
So humans will tend to favor the side tunnel in the beginning. Which can be easily protected by either side which means a sort of stale mate, while the aliens can infest the yard in the meanwhile.
Granted, the aliens have an advantage: their eggs act as repeaters and are available at S1, while humans get repeaters much later. So in S1 humans can only go as far as the yard entrance. Or move RC, if they're daring enough. But there are no good places to move it to in the yard except in the downstairs bunker, which can become a death trap just the same.