Because that is not what you are supposed to be using .invalid for. Like .example it is meant for local testing and documentation.
Additionally, using .invalid still causes additional things to happen – additional DNS look ups and queuing on the mail server for retries for one off the top of my head.
Using the "." format is supposed to cause an immediate hard fail. Causing the MTA to immediately stop attempting delivery. At least that’s the way the intro to the RFC reads.