Because an exemption for a dependent works like a deduction, not a credit. And the difference is important.
A deduction lowers your taxable income, which is then taxed at your tax bracket rate. So a dependency exemption of $4050 means there's now $4050 less income being taxed. So if you're in, say, the 15% bracket, then the amount of tax you would have paid on that income would be 4050 x .15 = $607. That's the actual dollar amount the deduction is saving you. So the higher your tax bracket, the more a deduction is worth in actual dollars.
A credit, on the other hand, comes into play after your tax has already been calculated, and it lowers that tax owed by $1 for every $1 of credit. And if the credit is refundable (some are, some aren't) then the IRS will refund any unused portion of the credit, meaning you get the credit even after your tax hits zero.
So, to sum it all up, a deduction never actually puts 100% of the deduction amount in your pocket, the way a credit might. And depending on the circumstances, a deduction may not make much of a difference on your tax return.
That's the reality of the situation.
