There's 'FP stacks' and "FP stacks" and some aren't expressly similar. Volumes of money/data get handled by FP stacks - Jane Street famously uses OCaml; Cisco runs their entire cybersec backend on Clojure; Nubank covers entire Latin America and about to spread into the US - runs on Clojure on Elixir; Apple has their payment system, Walmart their billing, Netlfix their analytics on Clojure; Funding Circle in Europe and Splash in the US; etc. etc. There are tons of actual working products built on FP stacks. Just because your object-oriented brain can't pattern match the reality, it doesn't mean it's not happening.