I need some sex ed apparently.

If someone has sex every day for 3 weeks straight, no sex on period week right?, and getting positive OPKs during that time... why wouldn’t they get pregnant? Why is it only a slim chance that millions of sperm aren’t catching a falling egg?!

Like can someone explain the scientific reasoning why there’s only a low chance of pregnancy happening even if someone has sex literally every single day of the month every single month?

(Health aside, as I have a two year old so I know it is possible!!)