Inlining is also about performance. Classes impose significant memory and code size overhead even if HotSpot does inline the code eventually .... but much code is still interpreted. And Kotlin also targets Android. Finding documentation about what optimisations ART does is difficult, but I imagine it's got a weaker compiler than HotSpot if only because it's newer.
Not directly related, but for completeness sake: Java 8 lambdas only generate classes for capturing lambdas, and even then, only at runtime (a class file isn't generated). Kotlin can't quite do that because it relies on method handles and invokedynamic, which are missing from Android.
Also, there are few compilers anywhere with optimizations as powerful as HotSpot :) And HotSpot compilation is only getting better and better. Java 9 will even let us use Graal as the optimizing compiler (i.e. instead of C2), which can do this: https://twitter.com/ChrisGSeaton/status/619885182104043520
