A quick Google search for "why do games ask you to press start" yields many insights.
Over at Game Development Stack Exchange is the following question: http://gamedev.stackexchange.com/questions/14182/what-is-the-purpose-of-a-press-start-screen
Several reasons are explored, the first paragraph of the accepted answer discussing a historical reason:
The historical reason for this is somewhat different from the current reasoning - previously it was to have a 'safe' screen to go back to which was always resident in memory, so that if the game had gone to a demo loop and the player touched the controller, it could instantly return to the start screen and let interaction commence with no delay.
Over at Arcade Stack Exchange is another question: https://gaming.stackexchange.com/questions/18780/why-do-console-games-require-a-button-press-before-showing-the-main-menu
The accepted answer discussing certification requirements:
I asked a friend who does certification for console games for a major studio (certification is the process to get it approved to be released on the console by the vendor). He said there's a requirement that the game must have some interaction with the user after a set time period, even if the game isn't fully loaded yet. The "Press Start" or what have you is to meet that requirement: the game only has to load that far within the time limit then the user can say when they're ready to load the rest of it.
And over at GamesRadar is yet another article: http://www.gamesradar.com/ask-gr-anything-what-are-press-start-screens/
In which they sought to answer the question, in part reading:
So we turned to our programmer friend, Martin Caine, who also helped us out with a question a couple months ago (Ask GR Anything: What is pausing anyway?). "On most modern games, that screen is used to identify the player," said Caine, who is lead programmer at Retroburn Games, which develops Xbox Live Indie Games like Positron and Accelerate.
Ultimately, the decision appears to be made with no effort to affect the User Experience.
