they just get old. every time you turn on the ignition switch or turn the switch to start the engine, you have electricity going through moving contacts. Sometimes when they are moving they will arc the contact points and wear the contact away. After enough time the contacts get burned away and the electric current will jump across the passing contact and build up a large spark. Enough sparks and the heat from those sparks will burn up and melt the switch.
there's not a lot you can do to prevent an ignition switch from wearing out. Make sure you have good connections to the battery. Making sure that your starter doesn't draw too much amps. Don't overload the circuits that go directly through the ignition switch.
over the many years I have been working in the profession, I have replaced more ignition switches in GM vehicles than, Ford or Chrysler. Ford, it's more headlight switches that burn out.