Most likely an immense existing app with some gigantic library of React components... I still work on a React app with class-based components so complicated that nobody wants to touch. Like, componentDidUpdate() is hundreds of lines of spaghetti code kicking off even more setStates. Even some of the newer hooks-based components are a maze of side-effects... effects kicking off other effects, triggering a memo, triggering another memo. It's a mess, and management does not have 'tech debt' in their vocabulary.