P.S. I built a wall.
Yes, every champion has a different role. The point of this post however, was more focused on how each type of champion scales with their stats and how power creep affects roles and stats differently.
For example,
power creep with AD items means that champions which scale extremely well with AD (zed, talon, riven, exc) on their abilities see the largest growth in power.
power creep on AP items means that champions which scale with AP get more ability damage, because abilities rarely deal consistent damage, due to cooldowns, it means you see these champions get more burst heavy, as their auto attacks have very little power contribution.
power creep on defensive items however are not unique to tanks, tanks really don't ever (or very rarely) get more than 100% scaling from items they buy, where AD gets at least 100% scaling.
So when riot goes out to make a general buff to tanks, like every role, they buff the items. However, the dynamics are different than how buffed AD items affect pure AP champions. Literally no champion in the game doesn't benefit at least 1:1 from defensive stats. It might not fit their designed role, but they can still utilize the stats.
To demonstrate this point, you consider an extreme case.
If riot were to massively buff tank items, say buff the stats so that they were 5x more gold efficient, then you'd have a situation where literally every champion would buy tank items. (Literally no champions would be excluded, but some would be ideal, generally champions w/ % damage)
If riot were to massively buff AD items, you'd see just about every champion buy AD items, except that there would be a massive discrepancy between AD scaling champions and AP scaling champions. (you could still see AD swain or AD ahri)
If riot were to massively buff AP items, you'd see that only the champions with AP scaling being played, this includes many tanks. (basically zed, riven, lee[probably] talon, and many marksmen would be excluded)
So that thought experiment shows how the stats are fundamentally different, now because of this, it means that to equate the stats would literally make the game out of balance.
In a mathematical sense you could say that each stat has a multiplier. Def x A, AD x B, AP x C. Each of these multipliers work differently (personally i'd guess something like this B > C > A), so if you want if you make Def = AD = AP, then the multiplier variables would mean that (Def x A) =/= (AD x B) =/= (AP x C). Unless you changed A, B, and C. Though to do that would be to uproot the very fundamentals of the game's design.