Why do people hate building armor/mr when the team have enough damage

Becasue it is nowhere near as effective as it once was.

Defensive items have gottne several nerfs to their overall stats. And while they also got cheaper the end result is that defensive items get outscaled harder than they did in season 2.

I miss you 1350 health warmogs, Force of Nature and Omen that slowed your attacker

Because players are selfish. There was several games I could have won and carried just fine except my teammates, who are playing things like Poppy, decide to build full damage instead of tanky and peeling. :/

Another reason may be key items with specific actives that are necessary.

Exhibit 1: Zhonyas

Because armor doesn't do damage.

The game has become all about dmg. Even tanks want to be killers.

What Irelia Bot said but there's also the part, where offensive items got overall stronger. Can't say much about AP part, as I am biased and from my POV it looks like it was nerfed if anything, but AD definitely got upped by a level.

On top of that Riot gave everyone true damage, so whatever you'd build to defend yourself, it's yet again weaker than it should.