I think this is a valid observation of the game. Of course, if it were to be changed, I think a significant amount of rebalancing would need to be addressed. If aliens were to actually hunt you, then this would pose a significant degree of extra challenge to the player, especially as the maps are so small. The activation method helps to mitigate this. On some of the maps though, you could quite quickly discover that you have 10+ aliens surrounding you. Maybe a compromise would be to have some types of aliens that are free to roam, and some that dont. Or to have aliens that only patrol a very narrow area, but are able to take shots at your troops the moment they spot them.
Good post

If aliens were to hunt you down, I can imagine the two small construction site maps becoming an absolute nightmare! XCOM forces wouldn't stand a chance unless the amount of aliens present were reduced significantly. And even if a significant amount of rebalancing were done, I think the result would be worse than the current design. With larger maps it wouldn't be so terrifying, but the missions risk becoming an overwatch drag; killing the pace of the game. Oh, and you'd have no incentive to explore the maps at all.
The only sensible instance of aliens actively hunting you would be during the nonexistent XCOM base invasions.
The aliens feel a little more active and actually present during terror missions--hunting down civilians.
I think a lot of it is just that the aliens will never get the first shot on you, making engagements terribly one-sided if you exploit it enough. Although they weren't terribly mobile in the original, they'd at least shoot at you if they had line of sight, even if you didn't. Combined with aliens just having longer sight ranges, and it made it at least feel like the aliens were somewhat active, even though they definitely weren't.
I think you are right, and I think
line-of-sight is a key issue. Aside from the line-of-sight bugs, there are a few problems: Unless activated, the aliens rely on the player's line-of-sight. Dormant spawns don't have any line-of-sight at all! This means one-sided engagements and no scary shots from within the fog-of-war. You also need mutual line-of-sight for every shot with two exceptions: Hunker Down and Squad Sight snipers (I'm disregarding Sectopods).
Taken together, I'll revise my critique.
Consider aliens having their own line-of-sight, and units being able to shoot (with a penalty, of course) at any enemies spotted outside the shooter's line-of-sight--provided there being no obstacles (e.g. walls). Add range penalties for both sides similar to the shotgun-type weapons depending on the effective range, and
fix or
improve the teleport pathing of patrols. I think this may improve the "Aliens vs. Humans" battlefield experience, and advancing through spawn points may not be so bad after all.
Sure, your squad will potentially be pinned down and fighting more aliens at once, but the aliens within the fog-of-war suffer severe aim penalties. And if the game becomes too easy with Squad Sight snipers, how about some anti-sniper Alloy Cannon Floaters?