Because once you get beyond the simplest runtimes maintaining compatibility between things becomes difficult fast and more complex runtimes have enabled a lot o

Because once you get beyond the simplest runtimes maintaining compatibility between things becomes difficult fast and more complex runtimes have enabled a lot of things people want, like actual portability between systems, JIT compilation, garbage collection, etc. Most language runtimes have native interop these days but it’s always a worse experience all around than using language-native libraries.