Schedule B is included on Form 1041 and it is for the Income Distribution Deduction.
As to why a separate schedule is not required to report interest/dividend income with Form 1041, the answer is because it simply is not required as it is for Form 1040.
In case you haven't noticed, there is also no Schedule A for itemized deductions to be filed with Form 1041 and Schedule D is different from the schedule filed with Form 1040.
