It's generally bad practice to sit idly at the start of any game without the timer running. Sometimes there are cycles already started by the time you gain control, it's all really silly. This game doesn't have anything like that in particular, but having a consistent standard across most games can be a good thing.
The argument about playing on mobile seems irrelevant, I'm not sure where that's coming from. If you have trouble starting your timer and playing right after skipping a cutscene or starting a file (on any game), try using a start delay and/or retime your run when you're finished.
Basically it's just the standard and it's good practice. If you know of a game that allows idling at the start after control is granted to the player but without the timer having started, you really should be questioning why on those games and not on this one.
Yeah, to echo Jumpyluff I very rarely see ANY game time it by first input as opposed to as soon as the game starts. They do exist, but usually there just isn't a good reason to do it that way.
actually, the bridge at the end of level 1 does move after starting the level, so gameplay would be slightly different based on when you start moving. I believe you also start slightly in the air, so your acceleration is slightly different from first frame vs later
