I think it's partly that when you ask for a 50% pay raise a year in theyre probably afraid you may tell all your colleagues who are your friends and who know your productivity and then everybody's asking for a pay raise.

Less risk of that happening if you are a fresh face starting out with 50% more. You probably wont share and plus, who knows what kind of hot shot you are?

Plus, your leverage at the point where you decide to take the job or not is at its highest. They'll infer that walking away is much easier for you than it is when you've spent 2 years somewhere and you're settled.