Please add [7z] or something similar to (r)dependencies of ark. P7zip is emerge --depcleand because of kde-apps/ark does not (r)depend on it and then it misses 7z archives support. Exact CMake message: CMake Warning at plugins/cli7zplugin/CMakeLists.txt:31 (message): Could not find the 7z executable. Ark won't be able to handle the 7z archive format.
$ grep -C 1 p7zip ark-16.12.1-r1.ebuild if ! has_version app-arch/p7zip ; then elog "For handling 7-Zip archives, install app-arch/p7zip" fi