Confused on due date 🤔

To determine your Pregnancy due date, why do we go off by the 1st day of your last period? If fertilization happens 14 days or so after. So say you do get pregnant, shouldn’t it go off by your ovulation day? It’s a big difference to me when you think you’re one week but according to due date predictor I’m 4 weeks? Can someone she’s some light on this? TY!