“In that day, he that shall be on the housetop, and his goods in the house, let him not go down to take them away, and let him that is in the field similarly not return back.”
The first illustration of the urgency of these days is to picture it in terms of escaping from catastrophe without looking back. Then there will be no time in which to go down and pack, or remove furniture (a common picture of escaping refugees), there will be no time to return to the city from the countryside. All will happen immediately. The point is not the giving of advice on what to do, but in order to indicate the speed at which all will happen. There will simply not be time for anything. And there is also the suggestion that they were not to have their hearts set on earthly things to which their thoughts would instinctively turn when they recognised that the end of all things had come (as Lot's wife did with Sodom). It is not a question of logical thinking, it is a question of what will spring into their minds at such a catastrophic moment.
Interestingly a similar picture is drawn of those who would be faced with the catastrophe which would face Jerusalem in 70 AD (Mark 12:14-18), a precursor of the final Judgment, words which Luke deliberately omits, possibly to avoid confusion.