Well, after it's developed it's got to be tested, bugfixed and smoothed out before it goes out. This is really the part that separates good game makers from the rest. GTA's on the realism train and people can either like it or not, but that involves making sure that some of those weird movement glitches you see pop up in IV sometimes. This is all the more complicated with the new engine, because not all movements are hardcoded and "canned" as they were in the older games. People don't fall the same way anymore. Have to make sure solid walls are solid, no texture gaps in the ground (or at least be hidden well enough so as to basically require someone to purposely go through, as to explore "blue hell"). Make final cuts to storyline(s). This is not always done quickly.