I haven't yet studied ART in detail, but I think one reason to compile at installation time is so the compiler can analyze the whole program, including the libraries provided by the OS. If just the application code were AOT-compiled to native code as part of packaging, the compiler would miss out on a lot of optimizations that are available to a whole-program-optimizing compiler. And note that JIT compilers basically do whole-program optimizations, such as devirtualizing and inlining methods, eliminating null pointer and exception checks, and others. So a unit-at-a-time AOT compiler would be worse than a good JIT compiler. But a whole-program-optimizing AOT compilation step at packaging time would require every application to statically link all the OS libraries. Compiling to native code on the device at installation time seems like a good solution to me.