Those dates usually aren't announced until close to when they are put onto the streaming services for the exact reason that you say. If studios released a movie on VOD and then announced "it will be on Netflix in exactly 2 months", no one would buy the VOD.

Triangle of Sadness will likely go to Hulu because of its distributor's (NEON) deal with Hulu. But, they are not consistent as to when Neon films show up there, and don't typically announce when they are coming until its already there.