The reason I think it is done this way is because of the cash flow. Your budget is broken down into weekly installments. If you turn a profit of $90M in a season, then don't spend it, why on earth would you just let that kind of money just sit there. A team owner would go blow that or upgrade something in the stadium or facilities. A franchise/business isn't going to sit on potentially billions of dollars. The game tries to be realistic, and letting a team stockpile cash for a decade then spend all of it in free agency one year down the line would be like finding a talking unicorn that has a magic wand. Maybe they can introduce owner personalities like in NBA 2K where you can see the financial aspect a little more, but just letting tons of money sit in a bank, I mean, c'mon.