Because our friend - Ayushraj has no idea what method will be required to improve missile accuracy.Not sure why a nuke would require 'thrust vectoring' or 'end game manuevering' to provide a high level of accuracy.
Unfortunately, even the most advanced inertial guidance system can't provide 200m level CEP, ICBM will need other tech assistance to reach that level.As long as a nuke maintains inertial guidance level accuracy (~200m), then its as good as it gets, since everything that it was intended to hit would be crispy fried no matter where it falls within the CEP area.
Basically, there are 3 key factors deciding ICBM or ballistic missile's CEP:
1. navigation systems;
2. precision of manufacturing;
3. number of testing launch;