There is a lore reason - it's called "Decadent Mastery" for gith pcs in Mordenkainen's Tome of Foes.
"Decadent Mastery - You learn one language of your choice, and you are proficient with one skill or tool of your choice. In the timeless city of Tu’narath, Githyanki have bountiful time to master odd bits of knowledge."
The lore reason is described under "Indolent Dilettantes" which basically describes that when Githyanki are not in war time, Vlaakith has to keep her subjects occupied because they are constantly craving novelty and stimulation. She encourages them to partake in arts and crafts more or less to distract them from pretty much getting antsy and rising against her.
I do think for Lae'zel, she in particular is quite the nerd, and seems to have really taken to reading in the libraries of K'liir. This is most likely the reason she knows about the zaith'isk but gith Tavs do not. Her chosen Decadent Mastery trait is probably Arcana.
