You guys just don't get it do you?
It's a path of least resistance the community will enforce usually on players they do not know, because ARC is the class you are least likely to perform horribly on to them. Plain and simple. It has been made the status quo. There are more than enough examples of groups beating Moogle with no more than a single ARC in party.
As for people who have a bunch of classes at 50, yet they chose to play one - you better be much better at what you do on that class you chose to play than the one that people want you to or else you're just messing it up for yourself. Moogle is NOT like what Batraal or Ifrit were. One class is not vastly superior.
During 1.18, ARC had a much higher DPS ceiling than the other DD. Plain and simple no matter how well you played, a decent ARC would do more DPS. That was wrong.
With Ifrit, LNC did the highest DPS, perhaps tied with MRD for single target at best, and it could completely avoid Sear. Obvious encouragement for class-stacking here.
Since 1.20, however, all the DPS classes are quite balanced. No, ARC does not do the greatest DPS either single target or AoE. It is however the most flexible and safe option. This is a great reason for it being preferred for groups of people who have little experience in the fight. You can shoot arrows at a safe distance and perform combos without positional requirements. It's much more n00b-proof. So when someone doesn't know you can perform, they'll tell you to go ARC.
BUT, since there is no reason why a competent player on another class shouldn't put out at least as much damage as an average ARC, if you chose to go in on PGL, MRD, or LNC then you best be doing just that - or else don't even argue and deal with it. First and foremost what is expected of each party member is to be doing anything they can to maximize their utility to best that encounter.
In conclusion, the classes are quite well balanced right now. People can perform more or less the same on any DD class in Moogle by design. The community, by practice, is enforcing ARC as a failsafe for people they don't know. This is not something the devs can fix, because class-stacking is not something the devs can fully prevent. They can simply do their best to not encourage it - which they have. There are lots of utility to other classes that most of the community is turning a blind eye to. Make some friends, play with the same people as much as you can, and prove your capabilities on your favorite class - yet if it doesn't work out and you are not able to make your favorite class work, then have the decency to switch to a class you can help the party with.