Some people faint, some don't

Fainting is a symptom of an inadequate supply of oxygen and other
nutrients to the brain, usually caused by a temporary decrease in blood
flow. Blood flow to the brain can decrease whenever the body cannot
quickly compensate for a fall in blood pressure.

here are some reasons I can think of :
Fainting may occur if the heart cannot pump enough blood to maintain a
normal blood pressure. usually it may happen to people with some sort of heart condition.

Tt may occur if blood volume is too low. An obvious cause of low
blood volume is bleeding. Another cause is dehydration, which may be due
to diarrhea, excessive sweating, inadequate intake of fluids, or excessive
urination

Also it if the vagus nerve, which supplies the neck, chest,
and intestine, is stimulated. When stimulated, the vagus nerve slows the
heart.

And it may also occur if straining reduces the amount of blood
flowing back to the heart.

As you can see most reasons are pretty dependent on personal anatomy and health state , for example In older people, an excessive decrease in blood pressure after eating a meal (postprandial hypotension ) may cause fainting

There are physical and psychological conditions that can make people faint, such as irregular heart rythyms and mental anxiety.

but... i've felt extremely worn out and ill before many times (sometimes worse than others) when i haven't eaten properly/had enough sleep and physically push myself doing a load of stamina excercise, and ive never really been that fit, just average/little unfit, and ive many times feel as though i was going to feint or wanted to, but it never happens. Can anyone explain why?