Why Do ADC Build Crit Chance And Crit Damage?

Crit combined with a good amount of AD and Attack Speed is generally the best build for lategame sustained damage output. The reason ADCs typically build this because their range allows them to survive with a glass cannon build, and because they generally have abilities or passives that provide other multiplicative stat bonuses that synergize with Crit builds (these abilities are called steroids).

This isn't a law of course. Some ADCs build lethality/armor penetration ( ). Some build for on-hit effects ( ). These are because their steroids or other abilities scale better with those stats and builds.

crit was ORIGINALLY designed to deal with tanks along with last whisper, then riot had a brainfart and made leathality which is the worst idea riot has had to date.

Crit scales really hard and adcs are meant to carry which means they need to maximize as much dps as possible

Because their champion designs are usually centered around high attack damage, high attack speed and crits as damage multipliers for late game. Their intent is generally a high, steady damage output later in the game

Auto-attack oriented champions tend to build crit simply because the chance for double the base damage is more profitable than their poorly scalling and cooldown-packed abilities. You could probably get additional 150 AD in the place of 60% crit, resulting in, let's say, 350 AD total. However, AD items usually lack Attack Speed, while crit items usually do provide AS. Remove bonus 150 AD and put 60% crit and one more attack per second there: considering you bought Infinity Edge as well (you almost always should), this puts you at: 200 attack damage, 2 attacks per second and 80% crit chance (+Infinity Edge's passive +50% crit damage) vs 350 attack damage, one attack per second and, let's say, 20% crit chance. Maths is easy there: within 2 seconds window of attacking, crit build deals 400 damage to the enemy assuming you don't crit at all. AD build deals only 350 damage in the same circumstances. Now, if you do crit, multiply that damage by 2.5 and keep in mind that your crit build has... four times the crit chance your AD build does. Also, considering Auto-attack oriented champions have abilities with poor - if any, - AD scalling, you're much better off dealing consistent AA crit damage. Crit / AS build is even more efficient on hypercarries like Vayne, or carries like Varus, who have innate on-hit effects attached to their auto-attacks. It's that simple, bro.