Why haven't the Doran's Items changed?

I actually think that's intentional.

Doran's items have always been about early game power at the expense of build paths and late-game power. The health and mana regen changes are percentile increases over a base value, which increases as the game goes on: 50% of 5hp/5 at level 1 is an effective 2.5hp/5, but 50% of 15hp/5 at, say, 10th level is 7.5hp/5...and so on. Percentile values get better late game as the base value increases.

But that's in opposition to the intended purpose of Doran's items, which grant EARLY game power that falls off later when other options become better. So a static X regen is really strong early, when a 50% increase might only give you, say, 2-3hp/5 instead of the 6 from Doran's Shield. But as your base value increases the advantage given by Doran's Shield decreases and eventually vanishes...which is exactly what was intended.

I would like to know as well but im gonna guess that it has to do with their early game focus whereas the other regen items were rebalanced for the whole game time.

Edit: read The Djinn's comment he took the time to explain much better and still beat me to it lol

dorans are not supposed to scale with the game.

Tripple incoming