The problem with releasing everything on launch day, is that there will be someone(s) who will find everything within the first week or two. They will release a video about it, and within the first month, everyone will have uncovered all the secrets. Then there would be nothing to do, and people would stop playing. It happens pretty often in many games. Skyrim did a pretty good job at hiding things, and people were still finding new things for longer than a month. But that is an exception to a pretty common situation.