Because the engine is not an interface. You can have then engine you want on any interface that supports it. I suppose the easiest way to play stockfish is to c

Because the engine is not an interface. You can have then engine you want on any interface that supports it. I suppose the easiest way to play stockfish is to click "Play with the computer" on lichess.org, you don't even have to signup! Then, you will find a /lot/ of GUI interfaces for your favorite OS (Linux, Android, BSD, Win/Mac...), and probably it will have Stockfish included. As such, you don't have to download or build stockfish, basically any free chess app that you will find will make it available to you. I can think of no reason to make a free chess interface without Stockfish.