They are rewritten in C++ already; Redpanda and ScyllaDB, respectively. Why waste the effort of rewriting it once again?
Prior systems I had built with facebook folly (c++ lib) and had also written my own eventing systems in the past, but the real value is having seastar being battle tested since 2016. Largely it has been the right decision for us as redpanda for it's young age has benefited from the stability of seastar.
I'm not necessarily advocating it [1] but the parent's claim was that those programs could benefit from memory safety, thread safety and better concurrency and C++ does not deliver along that axis.
[1] https://www.joelonsoftware.com/2000/04/06/things-you-should-...