I don't know why everyone just glosses over champion base stats in relation to items. There's a reason tank finished items are less efficient than damage items - It's to stop them completely running over the first 20 minutes. Here's an example.
Let's say maokai is level 7 and backs with 1.4k gold. He buys 2 cloth armour and two ruby crystals. At this level (without runes), he has 1025 health and 58 armor. Adding another 400 health and 30 armour is going to make him extremely tanky to physical damage, essentially having 2.8k worth of health.
Now you take a Jinx, at this point is probably level 6 (or 5) due to shared xp. At level 6 she has 70 AD, bases and spends 1.4k on a B.F, raising her AD to 110.
Before buying items at that point, it takes her 24 auto attacks to kill maokai.
After buying those items , it actually takes 25 autos.
That doesn't seem like a drastic increase, but at this point maokai is offering a lot to the team - He's able to initiate fights, provide CC and has decent base damages to allow him to start chunking whoever he catches. He also gets tankier to all types of damage, just from buying raw HP
Jinx however, who is only providing damage, is now less useful because she still can't get through maokai quickly enough to be relevant in a fight.
it seems trivial at first but it does matter. Finished tank items maybe be inefficient overall, but basic tank items are very efficient when being added to base stats.
Edit: I would like to see changes for tanks. I'd be happier if they had a small bump to overall efficiency for finished items and secondary items, but a reduction in efficiency for basic items. That would give them a small dip from their first back purchase, but let their first/second item spikes be more impactful.