Well, I'm sure the stupidity of people is one of the reasons. You just have to look at the example someone posted earlier in the thread where one of their employees was refusing to self-isolate for two weeks because "life's too short". You can imagine how people that dense would react to extended restrictions. Plus when you think of the potential for people to panic one can also imagine why it would be preferable to gradually ramp things rather than jumping straight into harsh measures. Though as you say, some of the government messaging seems to have gone too far the other way.
But even if everyone is was very calm and sensible, there's still only so long a country can last in lockdown conditions given the disruption it causes to pretty much every aspect of life.