Incumbent Presidents are hard to Beat?

The thing I keep hearing from pundits is "Incumbent Presidents are hard to Beat." Historical this is a rather questionable statement. After WWII , Eisenhower, Nixon, Reagan and Clinton were incumbents who won a second term. Bush I, Carter, and Ford all lost reelection (in Fords case election). Johson and Truman did not run for a second term. Leaving out Kennedy thats only 4 out of 9 winning a second term or about 44%. Now compare that to the 80-90% of members of congress that get reelected and "Bush is an incumbent" is not so overwhelming.