For three reasons: 1. There exist massive existing codebases in these languages, and simply the cost of a rewrite, retraining all of the engineers, and buildin

For three reasons: 1. There exist massive existing codebases in these languages, and simply the cost of a rewrite, retraining all of the engineers, and building new tooling is more expensive than making these languages run fast. In the case of V8, the cost for Google was to convince the entire web not to use V8. 2. These languages are in fact, the best tool for the job. The semantics of the language are the best tool for the job, and writing in another language is going to slow down the team (in the long run) more than making the language faster. 3. Opportunity cost is key when starting. I've seen this first hand with C* vs. Riak, and K8s vs. Mesos -- sometimes, choosing the language that lets you run faster, even at some sacrifice will put you ahead early in the game. You can figure out later how to replace all your duct tape, while dancing on the corpses of the competition.