Being president is a come down for Lex, who on his own time, runs his company like a fiefdom. Being president means giving up that freedom: all sorts of unsavory characters (elected officials, retired people, boy scouts) have claims on your time. And you can't very well hie off into whereversville for villainous adventure whenever you like, can you? You've got meetings, and press conferences, and peace in the Middle East to worry about.