I've been doing jvm apps for almost 20yrs...
What builds need to do and what people made the builds do are completely different things.
I don't remember a single project I was involved in that could not had had a simpler build...
The issue is that most people in jvm land are on a closed bubble and haven't seen anything else.
This is true for build systems as is for non OO design for example.
Most simply don't know better and the rest of us are simply stuck.
Ant and then Maven started simple enough but people always find a way to justify adding more stuff.
Gradle already started complex enough and they keep adding more stuff...