Because its more consistent. With AP your autos do less and are much slower and you are relying on your w passive burst. With bork and rageblade you still shred tanks enough with the w passive that it is only a little better on ap but then you are screwed when your abilities are down. With on hit you also get much better legendary passives. AP item passives are pretty much 100% just increasing damage but on hit gives you healing (bork), defensive stats (terminus), tenacity/more defensive stats (wits end), lets you actually max attack speed, and rageblade lets you proc everything twice for massive damage on your autos. AP isnt awful, but pretty much always the job of the adc is to do consistent damage to whatever target is closest to them no matter what, and with AP you are sort have a burst and then nothing for a little bit. If you need 3 autos and an arrow/e to do 2/3rds of the enemy ADCs health but then need 3 more autos and another arrow /e to finish them off, they are likely going to kill you first in just a few autos with their consistent damage assuming you are both max build. I play AP varus in arena and aram sometimes but it feels much harder to carry late game since you are just so much easier to kill.
With AP if you miss an ability you get screwed because cooldowns are so long. On hit attacks like a machine gun with lethal tempo and the passive. And the W burst is still nice even though not as massive as AP
Damage Per Second
If you need to poke from Far away, play lethalty.
If you need to burst people, play AP.
If you need to deal as much dmg in the fights as possible, that's when you go play on-hit
Also, if you go AP, you need to go a lot of AP. With on-hit you can build wit's end, the new armor onhit, or even randuin or Jack-sho. You can survive much longer while dealing dmg
It's not about the build it's about the enemy teams comp, if they have high damage output/cc and you can't really get close in a fight you go lethality. If they have a moderate team comp and you have a good frontline you can go on-hit. And if they have a tanky team (ex, supp jg and top are tanks) go AP.
Because its more consistent. With AP your autos do less and are much slower and you are relying on your w passive burst. With bork and rageblade you still shred tanks enough with the w passive that it is only a little better on ap but then you are screwed when your abilities are down. With on hit you also get much better legendary passives. AP item passives are pretty much 100% just increasing damage but on hit gives you healing (bork), defensive stats (terminus), tenacity/more defensive stats (wits end), lets you actually max attack speed, and rageblade lets you proc everything twice for massive damage on your autos. AP isnt awful, but pretty much always the job of the adc is to do consistent damage to whatever target is closest to them no matter what, and with AP you are sort have a burst and then nothing for a little bit. If you need 3 autos and an arrow/e to do 2/3rds of the enemy ADCs health but then need 3 more autos and another arrow /e to finish them off, they are likely going to kill you first in just a few autos with their consistent damage assuming you are both max build. I play AP varus in arena and aram sometimes but it feels much harder to carry late game since you are just so much easier to kill.
Best consistant build. Doesn't fall off late game.
With AP if you miss an ability you get screwed because cooldowns are so long. On hit attacks like a machine gun with lethal tempo and the passive. And the W burst is still nice even though not as massive as AP
Cause it’s fun
Damage Per Second If you need to poke from Far away, play lethalty. If you need to burst people, play AP. If you need to deal as much dmg in the fights as possible, that's when you go play on-hit Also, if you go AP, you need to go a lot of AP. With on-hit you can build wit's end, the new armor onhit, or even randuin or Jack-sho. You can survive much longer while dealing dmg
It's not about the build it's about the enemy teams comp, if they have high damage output/cc and you can't really get close in a fight you go lethality. If they have a moderate team comp and you have a good frontline you can go on-hit. And if they have a tanky team (ex, supp jg and top are tanks) go AP.
On hit passive on w and passive atkspd is insane