If it was reasonable to actually outplay and punish champions, despite being behind, then buying offensive items would be the correct decision.
However, given how little down time most champions have now and how low time to kill has become, buying a defensive item rarely means you will see a significant increase in time to kill when behind.
If AD scaling wasn't so high, that 150 hp vs 10 bonus AD actually gave a significant threshold of damage absorption, not just 1-2 extra auto attacks due to all the scaling that's associated with AD now, then you might have a point. Mind you that's before you consider that that 150hp already costs 50 more gold, and the fact that offensive passive abilities are substantially better than defensive passives on both items and masteries, you can see why it rarely pays off to buy defensive when behind.
Riot decided that the way they would lower the game time would be to make it so that fighting fire with water is less efficient and less consistent than fighting it with fire. They do it, because china, or specifically tencent, wants it that way.
[slayer-jinx-unamused]