It also allows DC to do more fine-tuning if need be, once the studio sees the reaction to those films.
There are seemingly countless DC movies in the works at the moment but announcing all of them now would be setting the universe up for another failure.
DC has overpromised and underdelivered before, so better to just focus on the next three movies hitting theater screens and go from there.
Focusing on Aquaman, Shazam!, and Wonder Woman 1984 shows confidence in them, and allows fans to get excited about those titles instead of getting too far ahead.