Not "How come". My younger brother and I have played it many times before and gotten a 100% fully complete map of Dune with all the Sietches' and Villages' locations. You've got so much free time to explore for the Villages once you've cornered the Harkonnens in the end game. Or while sending espionage troops for Chani. I've never missed a single Sietch before. Here's one that looks slightly incomplete, because we lost our screenshots of the completed map. Will have to dig around to find the 100% complete one.
Dune Sietch, Village and Fortress map. Looks slightly imcomplete from my knowledge.
