Why are companies like OpenAI and others that are all-in on LLMs still using ReactJS, Python and so on?

These programming languages and frameworks were made for developer convenience and got wide adoption, because it makes on-boarding easier.

This obviously comes at a cost of performance, complexity and introduces a liability into a system, because they are dependencies that come with a whole bunch of assumptions about how they are used.

Is this tradeoff even worth it anymore?