Retirement accounts other than IRAs are whole unto themselves. The taxable amount for any distribution from one of those retirement accounts is determined entirely from information known to the plan administrator.
On the other hand, determining the taxable amount of any Roth IRA distribution depends on information knowable only to you, information that is not knowable by the Roth IRA custodian, because the law treats all of your Roth IRAs as a single account even though your Roth IRAs may be spread across multiple custodians who are not required to share contribution and distribution information with each other. Since no one Roth IRA custodian has the information necessary to determine the taxable amount of any regular Roth IRA distribution, the Roth IRA custodian is required to report that they are unable to determine the taxable amount of a regular distribution.
