At least theoretically they could be however OCaml is in large part driven by Jane Street and has been for some time now and Jane Street's entire business model is built around optimizing for ultra high throughput, ultra low latency software where mistakes could cost on the order of hundreds of billions of dollars.

So my guess would be less that Haskell is not these things (nor couldn't it be) but rather that OCaml has had the external forces necessary to optimise for these things above all else.