I don't understand

I don't understand how people get pregnant "accidentally" or how they randomly have sex one night and it just so happens to have been on ovulation and whoops.. Pregnant.   If this was you, I don't mean to offend you at all.  But how in the hell does this happen!! Here i am using OPKs, checking cervical position and mucous, religiously using something called preseed, taking vitamins, doing stupid stuff like elevating my pelvis for 15 mins after sex and guess what... NOTHING.  I nearly have a breakdown every time I hear of someone who has fallen pregnant who was not even trying.  I realize some are more fertile than others but I'm about to give up. TTC is probably the most stressful thing ever when you want it bad enough.