It would just create a massive amount of work for modders for the benefit of basically setting up a rough list of workers. I've been going through and adding job histories and can already see why most modders leave them out as it is a vast amount of research and takes so much time to input.
It would basically require all mod makers to input the complete employment history of each worker up until the start date of the game. Again, many don't even bother due to the amount of time it takes to do so.
Employment histories don't account for face/heel disposition or gimmicks (and if it did that's creating a ton of more work again) so you'd still manually have to go in and set these.
It really only accounts for vague start dates (ex. January 2004) so you would still have to research who is on the actual roster
Not to mention the types of contracts (PPA, Touring, Etc), Promotions that aren't in the database (no one is going to want to create all of those, even if you could simply use placeholders), momentum, gimmick ratings, and other factors.
The contract function operates independently from the employment history function before the game starts (thus why you have to set an open ended Job History entry with the current promotion if you want the worker's start date set correctly).
As for the original question, if the mod is set up correctly and well researched, the contracts shouldn't have the start date of the game world (for example, all WWE workers in a January 2004 game shouldn't have a contract start date of January 2004) they should reflect when the person actually started/debuted on the roster. So if you're playing a 2004 game, Chavo Guerrero's contract start date should be March 2001.