Ever use Facebook? WhatsApp? Discord? Telephones? Docker? Amazon EC2? Bought something in a Walmart store or on their websites? etc. etc. Some of their core, critical parts are written in functional programming languages.
Just because you don't see FP out in the wild being conspicuous, doesn't mean it's not there ;-)
- WhatsApp: famous for being written in Erlang, a functional programming language, and processing more messages a day than the entire global SMS network with a team of 50 backend engineers
- Discord: handling extreme amounts of chat/call traffic using Elixir, another functional programming language based on the same tech as Erlang
- Telephones: telephone switches from Ericsson (and in fact, internet switches from Cisco) use Erlang
- Docker: core virtualization component for macOS (hyperkit) written in OCaml
- Amazon EC2: another core virtualization component of all computes, parts of the Xen hypervisor, written in OCaml
Well... chances are you've made thousands of phone calls through switches running Erlang. You may have used Emacs, Discord, Facebook, Pinterest, Spotify, Font Awesome, xmonad or visited web sites running Elm or an Elixir backend... or even driven a Volvo.
Here's a list of companies using Haskell, with similar links to other functional languages: https://github.com/erkmos/haskell-companies. It includes names like Facebook, Tesla, Klarna, Target and Kaspersky.
Not to mention the FP paradigms used in Python, C++ and other languages. In fact, if you've never come across FP "in the wild doing something" then I suspect you're living very much off-grid.
Not saying it's not out there or useful - just saying that after 20 years I'm yet to come across it once.