One of the reason why Lee sin and Elise are considered difficult champions is because of their very situational Item builds. For most champions you can safely run a generic item path while with these champions you have to build extremely situational every single game.
Now since it's impossible for me to list down all the situations you might face, I will tell you the rule that could be applied for most situations. (most not all!)
If you are fed -> Build damage, If you are behind -> Build tank
This applies for most bruisers and also for Elise and Lee sin.
Now the problem is that Elise and Lee drop off really fast. For those two champions in particular you should never buy more than 3 damage Items. (Unless you're super fed or you want to troll around).
The base damage is really high on both of these champions, but their scaling drops off kind of fast. That means that an early Bloodthirster/Hydra + Last whisper can be really strong on lee sing but building more damage after that won't make too big of a difference. Same goes for Elise with Rilay + Liandrys.
You are an assassin and that means that your main goal is to Kill the Squishies. Those 2 items will be enough to kill the squishies. Killing a tank does require a lot more items for champions like Lee and Elise and it leads to the risk of them being squishy and killed. Killing the tank is job of the adc, not yours. You only sometimes have to keep the tank away from the adc.