Let me read you the passive 
UNIQUE: Dealing physical damage to an enemy champion Cleaves them, reducing their armor by 4% for 6 seconds, stacking up to 6 times for a maximum of 24%.
Ok now lee sin is not a sustained fighter. He's a get in get out as fast as possible and do as much as you can kind of champion.
Marshbouy4 votesIt's the only good % AR pen item for tanky bruisers and he can still stack it pretty quickly with his combo so by the time he uses his R, he's probably at at least 20% AR pen.
Hammermancer1 votesyou can build FIVE of them
No but seriously the phage is great, the movement speed, the armor pen, the HP and 20% cdr are all good for him
AR URF1 votes[{quoted}](name=T3H UB3RN07,realm=NA,application-id=3ErqAdtq,discussion-id=eYEEuwLj,comment-id=0001,timestamp=2018-06-13T13:19:37.813+0000)
you can build FIVE of them
No but seriously the phage is great, the movement speed, the armor pen, the HP and 20% cdr are all good for him
Me personally I like 
rush 
after a few camps. build 
and than finish the game with
and then maybe build
Tegash1 votesShort Answer: It lets him exist after 15 minutes into the game.
Long Answer: Ok so yes Lee isn't a sustain fighter. But the thing is, he deals a lot of physical damage in a short amount of time. And it's mainly through autos.
Q + Q + AA + AA + E + E + AA + AA + W + AA + W + AA + Q + R + Q
Half way through all that you've already proc'd all 5 of Black Cleaver's armor reduction stacks.
If you build Lee assassin, yes it has huge impact early. But if the enemy team plays smart and stays in the game, Lee falls off and becomes useless. He Q's into the enemy team, and if a moderately fed ADC even sneezes on him, he's dead. Imagine him Q'ing into a full build Irelia? A Yorick?
Building Lee and then full tank lets him stay in the game and maintain his impact longer.
LilxPaprika1 voteslee hasn't been a sustained fighter for a while now.
the cdr on it makes sense for lee, his passive attackspeed bonus helps him proc the debuff faster too. If you can get a second Q off before BC's debuff ends on someone, it can hurt a lot.