Cuz the Shivans do NOT use such tactics. I believed that was already established.
Also rememeber the mission where we have the jugg actualy some distance from the node? You know the one where you get the chance to actualy take out its beams.
Also the shivans are a race which prefers an agresive tactic meaning attacking the enmi positions fleets and so on not a race that uses ambush, or anything like that. Also I do not believe I have ever seen a shivan warship bug out of a battle with the GTVA that just isnt in theyr nature. Especyaly the ambush tactic. Or rather I don believe they did. Cant remember exactly every move the shivans did during the game.
The Shivans
have used ambush tactics. Almost every initial encounter in FS1 was an ambush; of the Bastion when it sent in fighters to investigate cargo, of convoys carrying shield prototypes, of Lt.Ash in the cutscene, in destroying the Taranis. Not to forget ambushing the GTC Vigilant, and the Erinpura and the Vauban (Triton) when they travelled to the Warspite prior to the TAG test, or even the first encounter with the Ravana. an ambush is simply an unexpected attack and the Shivans have done that extensively.
In any case, if this tactic was as effective as you think it would be, do you really think the Shivans wouldn't try to counter-act or nullify it?
It's also worth noting badly damaged GTVA ships rarely bug out either, so it's probably not any reflection on the race but upon the realities of gameplay.