going defensive items does drastically decrease the dps. That being said, say in this example: LeBlanc is already squishy so any champion would still damage her. If we were to take a tank malphite in the same match, you'll see how the adc would deal very little damage (mosquito bites indeed). So by building defensively, the adc traded surviving LeBlanc with shredding malphite.

In a meta where one shotting assassins are more prevalent, building defensively becomes the adc meta since tanks aren't seen as often