Why can't I claim exempt status throughout the year?

Because its against the law!

The country needs money to run itself. Most taxpayers are required to pay as the money is earned - employees have it taken out at source and most of those who don't have to make quarterly payments to the IRS.

Think of it this way - how would you feel if your employer said "I'll pay your wages once we know how many hours you've worked during the year - all in one lump sum." Would you want that?