It’s possible to build applications that run ‘as’ the kernel, they need a bit of hoisting if you don’t want to make it yourself, libraries generally fix that, but it’s not entirely uncommon in the C++ (and Rust recently it seems) worlds. Even OCaml (a GC language) has such an embedded single-application kernel as well called MirageOS. ^.^
In general though they are for making highly compact container images for minimal size as of late.