Why does the military force soldiers to get promoted if they aren’t ready for the responsibilities or don’t want it?

SFC Christopher Smith
Their superiors believing that they have the potential based off of the superiors experience. There truly is no perfect way to know if someone is ready or not.

SFC Christopher Smith Who defines ready?
When I was a young SPC I was forced to go to the promotion board. I was a very good SPC (had won DIV Soldier of QTR, and was about to win SOY), but I did not want the responsibility of leadership. I did not feel prepared or willing to (potentially) lead Soldiers into battle and be responsible, literally, for their life or death.
And so I went to the promotion board against my will, and against my protests that I was not ready to be an NCO. My PSG and 1SG thought I was. They were right. I passed the board and went on to become a pretty damned good SGT (had the top organic squad in BN wide squad competition about a year later - as a SGT squad leader with two SPC team leaders).

Just because YOU aren't ready, doesn't mean you aren't ready, if you catch my meaning.

SFC Christopher Smith
I would agree with that as well. With each new rank brings more responsibilities and pressures. I would say it's not normal if you're not nervous at least a little.