Why can lookup field values not be mapped to system-defined fields in the child record? Ex) I'm mapping Contact info (name, phone, email, etc) via the lookup field to records in a custom module "Locations". Due to this limitation, I'm required to create a second email field to the record to map the Contact's email value to the record, rather than being able to map directly to the record's native "Email" field.

I'd rather use the system-defined "Email" field, and if an email unique to that location is also needed, use the system-defined "Secondary Email" field for that. I know this is a specific use case, however, I think it would be commonly desired, so I'd like to understand the "Why" behind this limitation.