I noticed the _dtCompleted field on the Certificates isn't available for Statements of Attainment as well, is there a field that does the same thing?

Yes, the same field exists for Statements of Attainment but it's called MAX_DATE. It contains the highest date of completion of the units listed. We called it MAX_DATE as it might not be a full completion but a partial completion (for partial completion Statements of Attainment).