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).
EEPROM programmers and various other ways of rewriting this VIN (turnkey tools using undocumented diagnostic commands giving you unauthenticated read/write access to memory) means it's just one insignificant expense for the chop shop (as it's amortized over all their cars) vs a multi-thousand-dollar investment in money for a legitimate vehicle owner or part-time mechanic.
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).