It's not quite as bad as the comments indicate.
Form 1040ES has been available since January (and IME has been similarly for all past years). It mostly uses the prior year (currently 2016) as the basis, but it does have the updated (2017) figures for items that are automatically adjusted for inflation: bracket points (and thus filing threshhold), standard deductions, Social Security cap, and maybe another one or two I missed.
The forms making up the actual return cannot be prepared very far in advance because, as commented, Congress frequently makes changes to tax law well after the year begins, and in some cases right up to Dec. 31. The IRS must start preparing forms and pubs -- and equally important, setting the specifications for software providers like Intuit (TurboTax) and H&RBlock -- several months ahead in order to not seriously delay filing season, and with it refunds, which nearly everyone in the country considers (at least publicly) to be worse than World War Three and the destruction of the Earth by rogue asteroids. I have 1040 series from the last 4 years still on my computer, and the download dates mostly range from late September to mid January. Although one outlier shows the range of possibility: 2013 form 1040 and Schedule A were tweaked in April 2014 because Congress passed a law allowing charitable contributions for Typhoon Haiyan to be deducted in the prior year.
Substantive, but relatively minor, changes happen every year, including many that keep recurring like the special (pre-AGI) teacher supplies deduction ("will they or won't they?"), section 179 expensing (changes slightly almost every year), and formerly the IRA-direct-to-charity option (finally made permanent last year).
As commented, the current Congress and President were elected on a platform with tax reform as an important element, and they are talking even more intensely than before about doing it, although whether they will actually do anything this year is still uncertain. However, if major reform is done it will almost certainly apply to future years only, and likely only start after a lag of some months to a year. They know it causes chaos for businesses and households alike to upend without advance warning the assumptions built in to current budgets and plans -- and IME as a political matter something that is enacted now and effective fairly soon but not now is just as good (but I think that part is offtopic).