We are very familiar with "Please fasten your seatbelts, stow your seat tables, put all window shades up, and make sure your seats are in upright position as we prepare to take off/land". Notice that flight attendants are very strict in implementing this even if you are the world's most travelled person. Why? They are all there for your own safety.
Safety is a serious concern when you will be flying at 40,000ft in a huge life support system which is an aircraft, in fact, going to outer space in a space ship and travelling in an airplane or no different. They are both life support systems.
Flight attendants are highly trained to provide that level of safety and security for you as any emergency situation may be fatal if everyone do not know what to do. Implementing standard procedures like asking you to stow your tray tables, keep seats in upright position, wear your seatbelts when fasten seatbelt sign is on are there for very important safety reasons and this is why we want you to obey your flight attendant.
This video explains the reasons behind each basic procedure.