For one thing, retrospectives often come at a time when everyone has been busting ass for a long time and they just want to go home. If you put it off till next week then it seems like another distraction.
Like the planning meetings, the timebox for a retrospective is often wrong. In some cases things went smoothly and there is a little to say. That is where you should be. In some places you budgeted 45 minutes but there really is a day worth of material.
On top of that in many (most?) firms there is no sense of psychological safety. Often the real message is that the process is 99% bull but you can't say that in a meeting which is claimed to be about continuous improvement but is really about how to rearrange the deck chairs on the titanic.
In his book "Good to Great", Jim Collins points out the use of the word "alignment" is a bad smell. In a healthy organization there is alignment, naturally, built into it. If you feel like you have to paint alignment onto it after the fact you are just causing more misalignment.