It's about standards, if you have 50 different projects each with their own different way of doing things, it becomes a nightmare to maintain the build system. Ant itself had this problem since it didn't dictate a single way of building / compiling code and eventually gave rise to Maven which forces you into a very strict process.
If I want to compile any project in my 200+ project directory, I run mvn clean package and I get a standard war file or jar file. Want to run it, I can either run mvn jetty:run or mvn tomcat:run, need to import the project into eclipse, run mvn eclipse:eclipse, want to run unit tests, mvn test, etc etc etc. This applies to my Java and Kotlin projects, old and new.
Although maven is very focussed on JVM languages, I have retrofitted a PHP project to make use of Maven, although I would rather recommend to sticking to the right build system for the job, Maven is heavy JVM focussed so not a good fit for PHP, Composer is a much better fit for PHP.
If there's a specific package management system for C#, I'd rather use that over Gradle or Maven (not a C# developer, so can not recommend anything). I personally find Ant too loosely defined - if I want to run your ant script, I need to read through the script before I understand what you're trying to do and what commands I'm supposed to run, maven I don't even have to look at the pom.xml, chances are mvn clean install will work out of the box.
By the way, I see there's a C# plugin for maven: https://incubator.apache.org/npanday/ If you have to deal with multiple programming languages, pick one build system that will work for all, otherwise pick the one that's best for your single programming language.