Look at that, two in a row
Gradle *builds* an app. running it is a side effect. Developer needs to do that often, user not so much. And long build times aren't Java specific, are they?
Of course the build system is separate from Java itself, who'd want to have to maintain the whole thing as one behemot? Even ages ago there was a compiler and a linker as separate tools.
One can always do everything themselves with javac. If one has the time to waste.