Others have mentioned pretty good reasons as to why they might be made that way.
From a story perspective, it could also be a way to showcase that the Githyanki are formidable opponents and not someone you take lightly... as you do come face to face with a Kith'rak. The fight is quite interesting to me, as it can go completely your way or completely sideways in an instant, but I still consistently manage to overcome it and I like that there are challenging fights in the game which can be beaten with a bit of creativity.
I always have Lae'zel as a Battle Master in my party, who gains 3 special attacks. The strongest I found to be are Disarm, Push and Riposte.
- Disarm Sarth Baretha and the Crossbow Gith, as they do the most damage and are completely useless without weapons
- Prioritize in quickly getting rid of the Greatsword Githyanki, as he will constantly spam Sleep to incapacitate your party
- If you have ranged, make them go up to the bridge via the ladders nearby. So that when any Githyanki teleports up there, you can easily shove them back down
- If you have melee, you shouldn't have any trouble if Lae'zel disarms the two Gith
- If you have pets, place them near the Crossbow Githyanki so he becomes Threatened which will make him miss more
There are so many ways to beat this fight, but these are pretty standard ways to do it.
