What happened on September 11, 2001, was simply a one-off incident. Yet after that day, security when travelling is very tight and there are many restrictions – most of them of which are not neceassary and too strict.
Never before has an event like that happened and it won’t happen again. Security restrictions are not necessary because there are very few terrorist attempts that have ever been conducted on aircraft. An event like 9/11 will never happen again.