there is nothing to be proud of about lacking extension methods. Kotlin has them and it allow to make amazing APIs that are much more ergonomic/reduce cognitive overhead.
I can tell you that if Java's language team encounters some language feature that solves a particular problem, they'd rather spend spend several years thinking how to avoid adding that feature while still addressing the problem, than the six months needed to implement it. A good strategy for doing that is to wait, and try to think of ways to solve multiple problems with one feature (even if imperfectly) rather than solve multiple problems with multiple features.
For Kotlin, extensions methods are essential otherwise you can not extend existing Java types with Kotlin types.
For C#, you need extension methods for LINQ because modifying the .Net runtime was not an option at that time.
For Java, changing the VM is not an issue, instead of extension methods you add default methods which can be overridden by implementations.
Same feature, different tradeoffs.