There are many packages that compile some kind of tools and then run them to produce other things necessary for the build, possibly several layers deep. In the

There are many packages that compile some kind of tools and then run them to produce other things necessary for the build, possibly several layers deep. In theory you can set up the build system to correctly know which things should be compiled native and which should be cross-compiled. Sometimes you even need native and cross-compiled versions of the same thing. In practice, this is a capability that few people use and even if everything is set up correctly at some point most people contributing patches do not think about or test maintaining the ability to cross compile and it gets broken. When you're building hundreds or thousands of packages for a distro such as Debian or Fedora this is a huge problem. Building natively in a full system emulator or on real hardware is the only way, in practice.