I bet there's concerns with users who don't have a robust mental model of what applications/windows are currently alive or starting up or whatever just losing track of a window completely if it doesn't grab focus, and getting annoyed at programs seemingly failing to start up at all (because they only create a window hidden behind a newly focused window).
One-size-fits-all woes I guess.
There's a bounce animation with the application icon in the dock, and then there's a dot next to the icon to show that the application is open. That should be enough for all users.
Bring back splash screens! If your application takes ages to open, throw up a nice little title card.