Why can't I map a custom field to a standard field?

Why can't I map a custom field to a standard field?

This would provide some useful flexibility.

For example, in Contacts you have a standard field called Department. But the Leads module does not have a Department field. I have department information for many of my leads (people working at colleges and universities).

I can add a custom field to the Leads module, but then it won't let me map that field to the Department field in Contacts.

Is there a way to resolve this problem so that I can import my leads and keep the department information?