You can add custom fields to every level EXCEPT the one that matters most.

Clients want detail about each line item. Line items for billable hours are tasks. I can do custom fields for date ranges at the project level, but if it's an ongoing project I'm editing those constantly.

If I have a line item that I'm generating out on a weekly basis, I want it to where the custom fields for week start and week end are automatically generating that text for the line item. I can only do that if those fields are at the task level.