Exactly? As in 'exactly the opposite?'
I don't think that the "default setting" of most games is that imbalanced. It takes either pretty dedicated effort or pretty crazy circumstance for things to be that imbalanced.
'Most games' of course, is a pretty broad sweep - most games are probably at least fair (even if they're imbalanced, the same choices are available to everyone).
D&D has never been so well-balanced that you'd have to work /that/ hard at breaking it, especially from the DM side. And in most versions of D&D, including 5e, you have to make an effort (rulings/variants/manufactured situations/'DM Force'/etc on the DM side, and/or restraint on the player side) or stick to a prescribed mode of play (6-8 encounter/2-3 short rest 'day' &c) to get it to much balance at all. Sure, many of us make that effort as a matter of course and after decades of playing the game we may not even notice it anymore, or, if we do, just consider it a fact-of-life rather than a flaw that could be corrected.
I mean, I suppose it may vary depending on what we mean by "imbalanced"
The working definition I prefer is maximizing the choices available while also keeping as many of those choices as possible both meaningful & viable. So, examples of 'imbalanced' would include having many classes of which only a handful are 'Tier 1,' or having many possible EL
n Encounters some of which would likely be rollovers for a level
n party, while others would be possible TPKs, and the rest a range between, with very few actually being the intended level of difficulty.
but the most likely meaning to me is having one PC that totally outshines the others.
That's a possible consequence. Another would be having a party where everyone is playing the same class with very similar build decisions, because it's just that much better than all the other possibilities (as opposed to because we just all feel like playing thieves).
When that happens, it's usually a case of one player knowing how to min/max and another not. That's like saying basketball is unbalanced because you put Lebron up against a 5 year old.
Min/maxers and new/casual players routinely play in the same groups, while 5 year-olds are rarely allowed to play professional sports. So that's a pretty bad analogy.
I think imbalance most likely comes from some kind of "flaw" in the application of the rules rather than the rules themselves. If the DM only challenges the party in certain ways, that's a big problem. Or it could be, at least, if not all PCs are designed to face that type of challenge.
Sounds like shifting it in a purely semantic way. The DM /could/ avoid imbalances by always running in a carefully-calculated way to spotlight each player's PC in turn, even though one of them can do everything the other two can, and one of the remainder is for more powerful than the other. That would mean using very different, tailored challenges, possibly resorting to implausible circumstances and even railroading. That /is/ always challenging the party in the same way, as opposed to running the world 'status-quo,' or just running whatever you like, as you could with a better-balanced party.
A rules system could lend itself to imbalance, but I think that it would still require help at that from the players and/or the DM. And I don't think 5E really falls in that category.
D&D generally has lent itself to imbalance enough for imbalances to occur as a matter of course unless the DM and players approach it 'just so' (something many of us have been doing automatically for decades, thanks to playing it so darn much). 5e is certainly less imbalanced than 3.5/PF (intentionally designed to 'reward system mastery,' which makes a virtue of imbalance). 'Balance' relative to the quixotic classic game more debatable. (Really, 5e and the classic game are presented in such a way that even definitively defining their systems in an objective way is iffy, let alone evaluating them for any trace of balance). Obviously, 4e is the outlier, being the most robustly balanced (though far from perfectly balanced - look at all the 'chaff' in the list of feats, for instance).