Probably because it doesn't really make sense. If you have to build multiple projects as part of your build, they should probably be in the same repository, or you should be consuming artifacts already created from other project builds.
For example with Maven if you run any task from the sub-project which depends on adjacent sub-project, maven will look for it in ~/.m2/repository. So you have to mvn install every time you're doing any change in any sub-project which is PITA when you're changing many sub-projects simultaneously (and if you did not mvn install, you'll have old version in repository and new sources in IDE and things go wrong way in a completely non-obvious manner). But if you're running maven commands from root pom, sub-projects will properly reference each other. So the proper way is to run all commands from root repository and specify -pl. But then you'll run into other issues, as not all plugins will work correctly this way (for example mvn exec:java).
With gradle it just works. You're running command for any sub-project and gradle will properly find any dependant sub-projects, build them, put them into classpath, etc.
