This is because the agency only forgives the tax debt in situations that justify it. With that in mind, the IRS rarely forgives the full burden of a tax debt. They could do that if you're really going through an economically difficult time. A transaction offer (OIC) is an agreement you make with the IRS that reduces your tax debt.
As your name suggests, you offer to pay part of what you owe to the IRS and the tax agency pledges to forgive the rest as a commitment. Under certain circumstances, the IRS will forgive the tax debt after 10 years. However, that 10-year period may be longer than expected, considering extended suspensions, the IRS tax settlement date compared to your last return, and whether or not you've kept up with your tax returns since the debt period began. When your tax debt is currently in a non-collectible state, the IRS will review your situation annually and, if your circumstances do not change, your debt will remain in this state until the statute of limitations expires, at which point the IRS will cancel the remaining balance.
Yes, in fact, the period of time that the IRS can collect a tax debt is generally limited to ten years, in accordance with the IRS collection statute of limitations. When the ten years have passed, the IRS is required to cancel the debt as a bad debt, essentially forgiving it. For an assessment of the tax debt on a tax return you filed (or on a substitute return that the IRS prepared on your behalf), this is the date the IRS recorded the amount of your taxes due and you can find it in your tax file. Whenever the IRS cannot currently collect any payments from you, and whenever you contact the IRS and wait for them to deliberate on your offer of a payment plan, or OIC, the 10-year period for paying your tax debt is interrupted.