Why do ADCs in ARURF scale up to over 3000 Base HP?

No health items, straight glass-cannon damage builds. 3k+ HP. If you're AP and you've built Rylai's, Liandry's, and Morellonomicon, the ADC still has more health than you do later on. None of it is from items or Runes/Masteries.

Problems I have with this:

1. The health itself really isn't that much of an issue - it's the synergy with their sustain. They have such a large base health pool that the value they get from sustain is astronomical and the massive pool of health that said sustain contributes to doesn't lead to diminishing returns and you can't do damage to them anymore. (Ie. an ADC with 1600 HP can't fully rely on sustain because they can only heal up to 1600 HP and run the risk of being bursted down. At twice that, the diminishing returns are massively alleviated.)

2. ADCs are really strong at every point in ARURF. Damage from abilities carries them through early/mid game pleasantly where they explode in the late-game.

Maybe it was deemed necessary. I don't know. And again, I'm not fully convinced the HP itself is the issue, but rather the synergy with sustain.

I'm just kind of sick of invincible Graves/Kai'Sa/Sivir that 2-shot everyone, running away with games.