Why can't bend the top of my thumb without bending the index, middle and ring finger in my left hand?

Deep flexors: Run in groups of 2-3 so that's why some people will slightly flex the other 2 fingers when you try to bend the thumb. If it's quite exaggerated, or didn't used to be a problem it may need a doctor examination to sort out whether there is an abnormal tendon,

Muscle function: I guess this is just one of the things you have to accept in life given that this is not really a health issue.