Building defensively isn't going to cover your trashy gameplay

If you lack the skill to avoid damage in the first place, trying the defensive route is always going to seem pointless since you're just face-tanking. I think this is a big reason why people keep saying building defensively is usually pointless.

Here's the thing, building a defensive item does not mean taking hits left and right yet still coming out on top. It means reducing the damage from that big burst combo the few times you actually get hit by it. Note my wording there, few. You're not supposed to be taking hits constantly in lane, you're supposed to actively play around an enemy while doing normal lane things.

If you can't deal with constant pressure from an enemy/put out pressure of your own, building defensively is just setting yourself up for a very one-sided attrition war because it's not solving anything. You're still taking repeated hits and will get worn down, how long it takes depends on the champion you're laning against and the person playing them.