This is not correct. It's calculated per pay period. Please see IRS Publication 15 and 15-T.
There's no good way around this either (to my knowledge). Other than maybe jacking up the number of dependents you have.
It is possible for employers to use alternative methods. IRS Pub. 15-T sec. 6 has details on common alternatives and constraints.
I have no idea to what extent employers support any of these alternative methods. I imagine it's limited to employers where much of their workforce is overwithheld using the standard methods. One example might be a school district where teachers are only paid when in session.
Deductions: 86% of tax returns use the standard deduction. It's OK for the IRS to assume that the standard deduction will be used unless the taxpayer tells them that they want to itemize.
Isn't this a 1 way street where the IRS wins?
For example, if you underpay don't you have to pay interest at a set rate for the whole year you underpaid where as if you over pay you'll get interest from April 15th until the time it takes to receive your refund which might be a few days or a couple of weeks?
For the average citizen, a little bit of opportunity cost is much less important than the costs of getting hit by a surprise bill, so it makes sense to err a little bit on the side of overpayment when filling out your W-4.
It's not always possible to file accurately that early.
A number of investment related tax documents don't come until March 15th at the earliest. Then you probably need to wait for your accountant to prepare everything, you review everything and finally you officially file it. For me personally this usually happens near the end of March. Realistically it can't be any earlier.
Also, if you're self employed chances are you're paying quarterly estimated taxes to avoid penalties. You're giving them money as early as a few months into the tax year. You also need to make sure you don't underpay or you can get penalties and you also owe interest. You either give the IRS an interest free loan by overpaying throughout the year or risk losing money.
It even notes on the form that you should update it if your financial situation changes.
You mention W-2, so I'm assuming the US.