Why is former presidents are still called president?

Former presidents are not called "Mr. President" - at least, not by those who remember their history.

Despite what you might see in the popular media, the correct form of address for a former President of the United States is to use the title of the highest post that he held before becoming president. This courtesy was established by George Washington; the only exceptions to this rule are for former presidents that go on to hold other posts after their administration has ended (Taylor, who became a Senator, and Taft, who became Chief Justice of the Supreme Court).

Thus, William Clinton should be addressed as "Gov. Clinton", as he was governor of Arkansas. G. H. W. Bush should be "Ambassador Bush", as he was ambassador to the UN. G. W. Bush should be "Gov. Bush", for his term in Texas. And President Obama will become "Senator Obama" once he leaves office.