There is good reason for this: theft protection. A long known crime model is the "chop shop" where they tow away a car, then cut it up for parts. Because the VIN is in the ECU a chop shop has less incentive to exist since there is less money in it.

It does of course make legitimate repairs harder, and probably there should be a process to write a new VIN in - but it should only be available to someone who can verify the ECU wasn't stolen (which isn't free).