Parents are responsible for their children - their health, wellbeing, ensuring punctual attendance at school, keeping them safe - and so have to ensure compliance somehow.
Some children are very attentive and compliant, and so asking nicely works. Some, like my own DC, are amenable for a lot of the time, but generally only hear things that interest them and often ignore the rest. Some will take a mile if you give an inch.
My DC respond best to simple clear instructions - "You need to brush your teeth please, then put your homework in your school bag and put your shoes on". If they're away with the fairies, it then becomes "Brush teeth, homework in bag, shoes on please". And then if they're still zoned out, "Teeth, homework, shoes NOW!"
Sometimes you have to be firm to be kind. My older DC does not enjoy being late for school. Hates it. But he'd while away the minutes at home playing with his lego or counting the cornflakes in his bowl. It's my job to keep him on track until he develops some executive function, and sometimes the softly-softly approach doesn't work.
The reason I don't talk to adults the way I talk to my children is because I'm not responsible for them, and because most adults don't require anywhere near as much direction as young children. They don't need to be told to stay away from the edge of the platform at train stations, not to talk in theatres or not to climb on seats, for instance.