You start lightweight so your body can get used to Pressing, Squatting and pulling three times a week. Concentrate on lifting with proper from meanwhile. This will prepare you for the heavyweight later. Starting too heavy will cause soreness. You will feel like skipping a workout often turns into two skipping workouts. Now you heave to restart and lost a week. This ruins your motivation and usually ends the programme.
Starting too heavy also causes plateaus. If you start with you five rep max today, you can not lift more two days later and another two days later. You will miss reps, get sore and end stuck. Your starting weight must be light so you have room to easily add weight for several workouts. The misconception is thinking lightweights do not build strength and muscle. Of course, lifting heavy is better. and you and you will work towards that with strong lift 5×5. But litter weights trigger your body to gain strength and muscle too.
The other issues with starting too heavy are that it encourages bad form. Instead of practicing proper form with easy weight, you must lift it at all costs to get your reps. This builds bed technique habits which will cause plateaus and injuries later when the weight gets ever heavier. Starting heavy is trying to accelerate your progress. You think it will make you stronger faster. But it does not because you end up missing reps, getting sore, skipping the workout and heaving to restart with lower weight.
Remember the fable to the rabbit losing the race to a turtle. You do not want to be the rabbit who starts heavy, gets sore/stuck and then has to restart. You want to be like the turtle- starting light, adding weight steadily, and getting there faster by avoiding soreness and plateaus on the way.