Because they stick first and can sometimes be reset by tapping them, eventually the weld themselves, but you want to have changed it long before that as it is dangerous. The barrel has a plate with three terminals on springs that rotate with the key making and breaking contacts on the rear plate, the chances of that internally shorting is more or less none existant.. True they wear and on Cessna's the plate and terminals are renewed every 2000 hours. The surge suppressor diode on the lower image was introduced to prevent arcing as the solenoid closed to reduce the chances of burning or welding the contacts as mentioned.
http://www.ercoupe.net/ads/76_07_12.html
