Since you asked, and it's a perfectly legitimate question, here's why it doesn't violate the Fifth Amendment (from Garner v. US):
The Fifth Amendment doesn't say "you can't be made to say anything that hurts you." It says "no person...shall be compelled in any criminal case to be a witness against himself." The only time Fifth Amendment protection applies is if you are being forced by the government to make a statement that could open you up to an accusation or conviction of a criminal offense. It's fairly broad (if it would help prove any aspect of the charge, you can claim immunity), but it's also restricted (you can't claim it unless it really would tend to incriminate you).
That means that you could only argue the Fifth Amendment if your tax return might help prove a criminal case against you (the fact that disclosing income leads to you needing to pay tax does not qualify as "self-incrimination"). But the amount of income is not one of those things that might incriminate you -- you can get income through all sorts of ways. The thing that is incriminating is revealing the source of the income, and you can claim Fifth Amendment privilege for that.
- The only time you could possibly claim privilege is if you were being forced into a statement that might tend to incriminate you in a criminal proceeding. A statement that makes you liable for tax doesn't mean it might incriminate you.
- Filing an income tax return isn't inherently incriminating. The fact that you have income not included in any other part of the return (illegal income goes under "other income") doesn't imply you've committed a crime -- lots of people have other income.
- The amount of income can't be incriminating. The source can be, but you can claim Fifth Amendment privilege for that if it might incriminate you. And lawful income under "other income" still might give rise to a reasonable fear of prosecution, so the fact that you're justified in claiming Fifth Amendment privilege in source of income doesn't mean you're guilty of a crime.
Note that there are cases where you are flat-out exempt from filing tax returns under the Fifth Amendment: Marchetti v. US and Grosso v. US found that registration and tax on gambling could be blocked by a Fifth Amendment claim, which didn't even have to be asserted at time of filing, because merely filing the special return would establish you as a gambler (heavily regulated/often criminalized at the state level). The difference with the normal income tax form is that everyone (just about) files one, and so filing it doesn't mean you're a criminal. If there was a separate line along the lines of "Income from Illegal Drug Sales," that might be one thing (anything other than $0 is inherently incriminating). But all the questions are broad, and have many legal sources of income associated with them.