Because you need to use river sand for aggregate, if you salt water sand the concrete will crack.
Lousy or songlap contractor will use "washed" sea water sand, but it is never clean, that's why you see a lot of building with crack or compromised concrete.
Malaysia all river sand kena exploited and sell either to Singapore or China company.

No problem with plastering, maybe they are only shrinkage crack, not structural unlike concrete.
That's why Cantonese say shoddy construction as "ham shui lou" - salt water building - earlier cases uses salt water sand as aggregate, not only concrete crack, the steel bar inside also corroded.