Because having to attach headphones only to not use them as headphones makes for a stupid-looking user experience. Most users don't comprehend "antennas", much

Because having to attach headphones only to not use them as headphones makes for a stupid-looking user experience. Most users don't comprehend "antennas", much less using headphone wires as such. Prompting for headphones but using the speaker would result in most users coiling up the headphone cable (in bewilderment) which pretty much defeats the point of having an antenna. Never mind the fact that not all users would use headphones not suitable for use as an FM antenna (too long or short, wrong impedance matching, etc).