Can a single file tax as "married file jointly" to take advantage of double the standard deductions? Why or why not?
As the other answer states, tax returns are filed under the penalty of perjury. This is governed by the 26 USC Sec. 7206 (Title 26 of the US Code is also known as the "Internal Revenue Code", or IRC):
(1) Declaration under penalties of perjury
Willfully makes and subscribes any return, statement, or other document, which contains or is verified by a written declaration that it is made under the penalties of perjury, and which he does not believe to be true and correct as to every material matter; or
shall be guilty of a felony and, upon conviction thereof, shall be fined not more than $100,000 ($500,000 in the case of a corporation), or imprisoned not more than 3 years, or both, together with the costs of prosecution.
However, that's not the only penalty, and in fact not the main. The perjury is just signing it. But submitting it in order to not pay tax you should be paying would be fraud, and that is governed under the IRC Sec. 7201:
Any person who willfully attempts in any manner to evade or defeat any tax imposed by this title or the payment thereof shall, in addition to other penalties provided by law, be guilty of a felony and, upon conviction thereof, shall be fined not more than $100,000 ($500,000 in the case of a corporation), or imprisoned not more than 5 years, or both, together with the costs of prosecution.
In additional to the criminal penalties (up to 3 years in prison for perjury and up to 5 years in prison for fraud, and the $100K fines for each), there's also additional tax penalty for fraud under the IRC Sec. 6663:
(a) Imposition of penalty
If any part of any underpayment of tax required to be shown on a return is due to fraud, there shall be added to the tax an amount equal to 75 percent of the portion of the underpayment which is attributable to fraud.
So in total, you may end up sentenced to 8 years in prison (3+5), a fine of $200K (100K per felony), and 75% additional tax on the amount you were trying to evade (in this case the difference between the standard deduction for single and MFJ).
Given that to file MFJ you'd need to report the tax details of your spouse who doesn't exist, I'm sure there would also be identity theft charges and what's not as well.
So I suggest you think twice.
Can a single file tax as "married file jointly" to take advantage of double the standard deductions?
It is of course not your correct status, so that would mean you would face fines and possibly jail time. In addition becasue this saved you money by underpaying your taxes, you would face interest and penalties which could be substantial.
Why or why not?
Your assumption that married people save money on taxes isn't entirely correct, and the only way to save big time money is to invent a person.
The first obstacle would be that you need to have a social security number or tax-id number for this spouse. That SSN would have to be real, and match the name of a real person. That SSN would also have to be unconnected with another tax return. They couldn't be the taxpayer, spouse, or dependent on anther tax form, without triggering the attention of the IRS.
The other approach is to fake the marriage. Now if you were not married, but did have a person who cooperated by joining you in the tax fraud, they would also face the same penalties that you did. They would face these penalties because they would have to agree to not to file their own tax return.
Also don't forget to file the same fraudulent type of return with the state, because if you don't the discrepancy will eventually catch up with you. The state will also want interest, penalties, fines and jail to go along with the federal charges.