Jane Street's use of OCamel gives them a comparative advantage in hiring. Many devs put a premium on the tools they use. Same story with Anduril/Mercury using Haskell. You can run a successful prop trading firm/weapons design shop/startup-banking(?) platform without these things. They differentiate you from your competitors by being cool, not by being the best technical solution. If anything they were otherwise unprove at scale and out of pure engineering consideration a gamble. Developers are consumers.