Actually, there is. The devs can balance the jobs so their overall contributions are close enough no one cares. You keep insisting on blaming the community yet if there will always be someone locked out. Why isn't Warrior? It is undeniably the weakest tank yet you will never see anyone hard lock PLD/GNB which has become the de facto "meta". Why? Because the difference between all four is negligible. For argument sake, lets assume the highest rDPS possible is 17,000—which is achievable on Black Mage and Samurai, or thereabouts at least. Now lets assume the lowest—likely Dancer—brought somewhere around 16,500. Congratulations. No one cares what job you play anymore because they're all close enough it wouldn't matter. Now obviously this are just random figures I pulled but it helps illustrate the point.
You only see jobs being locked out when there exists a sizable imbalance. It's incredibly disingenuous to fault the community when, pragmatically speaking, you will have an easier time if you lock out certain jobs. What incentive does a random PF group have to bring Red Mage over Black Mage when the latter can outright die and still contribute more damage? If the only objective is to clear and nothing else. Then it stands to reason you want whatever advantages you can have to help achieve that. Taking Dragoon, Monk and two Black Mages (or even a Range despite them being weaker) makes it far easier than if you ran Samurai, Bard, Red Mage and Summoner. The latter is diversity for diversity's sake. There's no reason to take that comp beyond being nice because it's widely inferior to the one boasting the big three.
None of this touches on the fact it's a demoralizing feeling to know your preferred job brings such low damage relative to its counterpart that you switching, even if you were weaker on said job, would help your team more. A friend of mine is more than a little frustrated that all his efforts to reach 99% on Red Mage can be outdone by a slightly above average Black Mage. The same can be said for another friend who feels pretty crappy when I die and still out-DPS her (or almost) simply because I play Dragoon and she plays Dancer. In fact, just to emphasise how absurd the discrepancy is. I died two weeks ago in E3S and got a 9%. She didn't and got 94%. There was a difference of 400 rDPS between us. And I had less gear than her.
Is this ridiculous imbalance really a hill you want to die on?