That's a great way to describe how it works in real life; it's a terrible model for a fantasy roleplaying game.
Everyone says they want "cultures" and "backgrounds" and (yes, even) "races" to be more flavorful and iconic, but then they backpedal and start talking about individuals and exceptions and special circumstances and then they wonder why everything keeps coming out the same dull, flavorless shade of beige. They're homogeneous because you keep homogenizing them; if you want things to be special and different, you actually have to be willing to let them stay special and different. You have to be willing to say that they're not the same thing, that they're not interchangeable, and that they're not something to be ignored and that they're not 'guidelines' to be reskinned whenever someone says they want to play "a dwarf, but not like all the other dwarves".
It is literally the same people complaining that "fantasy races" are just different rubber forehead aliens that refuse to let them be anything else. When everyone, every single instance of something is 'an exception to the rules', nothing is special-- everything is blandly, identically unique, just like everything else, and it's boring. When you mix everything in the kitchen sink together, the only flavor you're left with is dishwater.
For my part, this is why I have always spoken for
key racial features.
Dragonborn have dragon breath, faster healing, and a frenzied edge when the chips are down.
Tieflings have infernal wrath (what 5e diminished to merely
hellish rebuke), amped-up damage against weakened foes, and the other side-effects of having infernal blood.
Eladrin--"high elves"--have teleportation, Trance, and resistance to mental effects, as well as breadth of training.
Elves--"wood elves"--have deadly accuracy, help their allies to perceive more, and navigate wild spaces with ease.
Etc.
These are things that actually communicate something about Elf-ness and Eladrin-ness and Tiefling-ness and Dragonborn-ness. They communicate the physiology. When coupled with further stuff that actually
does reference culture--Arkhosian, Turathi, Cendriannic, Nerathi, etc.--you can get a pretty cool understanding. Most dragonborn will be Arkhosian (or, rather, "Arkhosian-in-exile"). Many Tieflings, on the other hand, probably don't put nearly so much emphasis on the culture of Bael Turath, being more caught up with current-day stuff. They haven't mourned their empire quite the same way the Dragonborn did.
And yes, this is specific to PoLand. I get that. There are plenty of things you can do just as easily in FR (Dragonborn from Tymanther, Tieflings from Elturel, elves of various sorts from Myth Drannor or the like, etc.), and the almost-guaranteed upcoming Dark Sun book can have cultures from each of the various city-states. Point being, the PHB things are
way WAY inadequate if the point is to express actual culture through 5.5e backgrounds. Speculating about books that haven't come out yet, or responding to books that literally only JUST came out, won't be factored into this because...I literally could not have had any chance to learn from/about them.