The reason everyone builds pure damage

Because these champions are not only allowed to build pure damage, but are **encouraged **to invest none of their gold into mitigation stats, and the multiplicative nature of power growth in the game means that your damage will be rendered meaningless, long before theirs is meaningfully impacted by your defensive investment.

And before someone starts trying to make some kind of chicken or egg argument. No. It's undeniable that the assassin class, as it's designed and balanced today, came well into the game's design. In fact, many of these champions, particularly the older ones which have since been reworked, were originally considered some form of fighter and/or mage.

I'm sure there's other champions who share similar problems, but that's literally just the list you get when you filter by "assassin" on the wiki. Clearly this doesn't account for champions who can easily build as pure damage and play like an assassin. Such as or