Why is the drive through only an ‘outline’ plan?

As part of the justification for selecting an out of town centre site, the developer asserts that the drive through plan cannot be “disaggregated”, or considered separately.

The argument is that a drive through needs vehicles to have easy access and flow through the site. Therefore there is no commercially viable town centre location for the development.

If the drive through cannot be disaggregated by the planning decision makers, then why has it been disaggregated by the developer, and submitted as an outline plan only?

Surely for a decision to be made on the application then the full plans for the drive through must be included?