Because the calibration points for both scales are based on the state changes of water.

At standard pressure water melts (changes from solid to liquid) at 0°C / 32°F.

At standard pressure water boils (changes from liquid to gas) at 100°C / 212°F.

Since both scales are linear, 50°C, which is the midpoint between 0°C and 100°C, is equivalent to 122°F, the midpoint between 32°F and 212°F.