Doesn't your taxable income also have to be X amount before you actually have to file quarterly?
Yes and no.
If you are self-employed (including being the owner of a sole proprietorship, or in a partnership, LLC, or S-Corp), then you have to file
if you anticipate your tax liability to be more than $1,000.
If you are a regular corporation, then you have to file quarterly if your tax liability will be more than $500.
In those two instances, it's not based upon how much you make in gross income as much as it is based upon your taxable income and the ultimate amount of tax due.
Aside from self-employed individuals, other people may have to file estimated quarterly taxes if the amount they owe will be more than $1,000
and their withholding amount is either A) less than 90% of the total amount that will be owed, or B) less than 100% of what was owed the previous year.
Basically, they try to give employed individuals the opportunity to correct their withholding amounts so that the vast majority, if not all, of the taxes are paid throughout the year with the withholding deposits. If you are not withholding with a W-2 (self-employed) or otherwise are not withholding the proper amount, then they will require you to pay quarterly.