Why does everyone on the boards treat adcs like they start at full build?

Every time I see somebody list an adc build, it uses 6 items. This is really misleading because not only are adc items relatively expensive, so it takes time to get going, they're at their absolute peak at full build. Of course they look overpowered when you assume that the adc is at full build. That's the entire point of a crit build. These builds are meant to be strong at full build.

But most games won't allow the adc to get to that potential. You shouldn't be letting that adc get to 6 items.

Lifesteal will of course be good when an adc is at their full potential. Lifesteal is at it's strongest when you maximize your autos, like a crit build.

If you really want to guage the strength of a build, you also need to consider what it takes to get to that build. If an adc goes for 3 crit items right off the bat, they don't have armor pen, lifesteal, or defensive items at that time. If an adc goes for lifesteal, they delay their crit items.