Deer & Screams...

When humans and some other species scream or make loud distress sounds, others of their kind come to help. But when a predator attacks a deer or some other similarly vulnerable species, those particular animals receive no aid from any source.

What then is the evolutionary disconnect that defines the biological need for some species to scream amidst danger when they will never get help from anywhere?

Rare coincidences are on record, but nothing definite enough to establish a eco-guardian, so to speak, for deer and select other beasts. Did they, at some point, actually have a protector species that would, if present nearby, come to their aid? What does that mean for all the creatures we know of today that lack such a symbiotic bond?

Then again, if such symbiosis is strong enough, wouldn't it drive the relatively fewer predators to starvation and extinction? So, the initial question still remains: why dah faq do deer scream?