Why does Boris keep saying..

Because it is true and many people in this country never follow the news so it can take them months to get the message. I remember about 4 weeks ago getting a haircut and the barber had not heard a word about coronavirus, even in China.

If everyone followed these rules for the rest of their lives there would be no more common colds, influenza, measles, mumps or any other infectious disease but a few people won't.

He calls them 'the rules' not 'his rules'
He is indicating that he is not the expert and the rules are imposed by others
If the figures and advice turn out to be some degree of hogwash he can then distance himself from what was advised.

It isnt true though is it? Flattening the curve means it takes longer for things to get back to normal.

Yes it protects the nhs, and yes it indirectly saves lives but it doesnt return things to normality quicker?