Are you a slouch potato? Sit up and listen!
There is something called organ comfort. There are various aspects to this. Just to handle one aspect of it - see right now, most of the vital organs of the body are in the chest and abdomen region. These organs are not rigid, they are not fixed with bolts and clams. They are loose, hanging in nets. Only if you sit with your spine erect, your organs will be in the maximum possible comfort.
Keeping the body erect is not because we don’t like comfort, it is because we understand and experience comfort in a completely different way. You can train your muscles to be comfortable, with your spine erect, but you cannot train your organs to be comfortable while slouching. There is no way to do it. So, we choose to train the body, so that our skeletal system and muscular system is comfortable sitting this way.