Why shouldn't you buy defensive items on melee damage dealers?
I totally get it when talking about adcs or mages. Those champions have range, they usually don't need to be durable.
However on champions like Diana, Briar, Gwen, Jarvan IV, and so forth, I worry a lot about CC and burst damage. When I get a couple of kills on those champions and finish my two items, like Lich Bane+Shadowflame or Profane Hydra+The Collector, I feel tempted to buy a big HP+resist tank item, because at that point the enemy champions finish their first damage items too and they group up more, which increases the likelihood that I'll get hit by CC and eat a bunch of damage.
Am I wrong to feel that way? If I look at Master+ rank builds, it is always an "either/or" thing. If they buy big damage, they never buy HP and vice-versa. Don't get me wrong, I've seen games where the glass cannon builds pay off and they never stop snowballing, but I've also seen games where they get repeatedly nuked and throw the game, like when a fed full AP Diana keeps getting hit by Ashe arrows or Syndra QE and then dies to a couple of basic skillshots without accomplishing anything.