Well, because why not? Sett's W and R scales with HP meaning that your two most powerful abilities are made way stronger than normal. Additionally these items and runes make sett one of, if not the the tankiest champion in the game that can literally oneshot squishies and in rare cases the entire enemy team. And his already INSANE lategame is further enhanced by our scaling runes and powerful items making him nearly unkillable in teamfights let alone 1v1.
For example, lategame you can easily reach upwards of 6500+Hp and if you have a team with at least 9 braincells distributed amongst them there is no way the enemy team wins the fight if you tank 5000+damage without taking your W-shield, healing and astounding amounts of damage into account.

POWERSPIKE: YOU REACH AN INSANE POWERSPIKE AT 100 BONUS AD AS NOW YOU USE 20% MORE OF YOUR GRIT FOR DAMAGE WITH W
EARLY GAME YOU ARE WEAKER THAN THE AVERAGE SETT SO DON'T LVL1 UNLESS YOU ARE 100%SURE YOU CAN WIN