Out of necessity. Not in the interest of individuality. Members of the Horde took an oath to subsume their individuality to the will of the Warchief, who was considered to have ultimate authority in all matters. Alliance take no such oath.
Even if it is the case that the Alliance is not individualistic, that does not make the Horde individualistic or moreso than the Alliance. Thalyssra would find no individuality in subsuming her people’s will to that of the Horde’s will. I mean Thalyssra, Valtrois, and the others even got roped into a war they didn’t really want to have any part in to kill their erstwhile cousins - the very same people that helped them liberate their city btw but I suppose it’s warranted because they were just not very nice - and do morally ambiguous things that they had no desire to do. So much for individuality. What do you think would’ve happened if Thalyssra said, “No, we’re not doing that” to Sylvanas? So either Thalyssra is a hypocrite when she said our people will not be conquerors, or she’s dumb.
Also how the hell would she know what either the Horde or Alliance is like? Her people have been in the same spot for 10k years and have known representatives from the other factions for less than one.
Because they got over it already. During Cataclysm.
You’re really only hurting the case for your theory because if the Night elves could forgive the Shendralar then they could forgive the Nightborne. Tyrande’s prejudice makes no sense and like everything else in WoW lore is contrived.