No, we have not 'just decided' - its been guidance for decades, and as mentioned, its becuase of how people react to alarms in that occupancy, not just down to vandalism or milk, toasters, deoderants, showers or spray polish, and as such a general fire strategy was based around how people behave. As mentioned in another thread, most fatalities in flats are within the flat of origin, so there is no reason why this should be altered.
i'm all for spending money on systems if they have a significant contribution to the safety of the building, but common alarms don't (in buildings with an appropriate compartmentation strategy), so a further cost like time delayed smoke alarms are a solution to a problem that does not really exist.