[$] Old compilers and old bugs
The kernel project goes out of its way to facilitate building with older
toolchains. Building a kernel on a new system can be enough of a challenge
as it is; being forced to install a custom toolchain first would not
improve the situation. So the kernel
developers try to keep it possible to build the kernel with the toolchains
shipped by most distributors. There are costs to this policy though, including
an inability to use newer compiler features. But, as was seen in a recent
episode, building with old compilers can subject developers to old compiler
bugs too.