I believe the theory is that Thargoid technology has "fossilized" - they advanced to a point where they were far superior to any other race they encountered, on their home planet and elsewhere, and they've been that way for millions of years so didn't need to "advance" any further. They have tech that works, and see no need to improve it or find better alternatives. Akin to the hive insects of Earth ecology (ants, termites, bees): they have "technology" such as farming, engineering etc, as good or better than anything Humans could create on a comparable scale, and they've had this "tech" for millions of years, but it's all instinctive; there's very little selective pressure to build a better hive, or a better farm, so they stick with what they have. Under this theory, the Thargoid "hive minds" aren't creative enough to say "hey, let's try and build a better spaceship", until and unless there's actually a need for one.
It is the key Thargoid weakness: the trade-off for the high efficiency of a eusocial species is low adaptability. It's a fairly standard sci-fi trope for insectiod alien races. The Thargoids are following their "programming" doing exactly the same thing to us as they did to the Guardians, and to whoever else they might have exterminated in the deep past.