Even for building and customizing we need to save as the rules into application specific rulesets, as the UI-KIT ruleset is locked. so why not kept it in the PRPC OOTB application. Im i missing anything here?
The UI-Kit is added by Application Express when creating any new application. If it were added to the PRPC OOTB application, it would be added to all applications, including ones that weren't designed with the UI-Kit in mind.
This is a precaution to prevent unexpected changes to applications when upgrading to a version of Pega 7 that provides the kit.