There are ways to actually prevent hijacks in some cases. In other cases, you just need to detect and mitigate. Let's suppose that somebody is setting up an espionage campaign against you. Would you rather have this campaign last for 25 minutes or 25 days? If you can stop it after a few minutes, or after half an hour, say, it's much better than letting them eavesdrop on you for weeks. We've seen attacks that have lasted for weeks and even longer.
Some types of attacks are very short. By the time you detect and try to mitigate, it's already lost. Many of the attacks, especially the ones that are sponsored by government agencies, can last many weeks.