Because it wasn't added to the game as some sort of innovative feature to shake up the industry. The system to change class easily was simply copied from FF11, the other MMORPG. It's that simple.
I'm sure they must have had a reason. You can change it in Bozja and Eureka and I think you used to be able to in Diadem but I don't know now.there is barely content you can change your job in. now this makes sense in a lot of content but not everywhere. in the new special dungeon you can change your job... but you get a looong cooldown and therefore boring waiting time if you do that.... for no reason.
You can't change it in dungeons because the code there expects a certain composition and it completely breaks down if you have two tanks or two healers. In one of my 10,000 dungeon runs it got bugged and whoever held a certain tank or healer spot (I forget which) got duplicated so that we would have 5 people in the party. It's designed a certain way and just breaks down if it's changed.
That is true and a philosophy SE held for a very long time, but it began to change with the release of Shadowbringers and especially Endwalker. I see a lot of jobs within categories getting similar types of playstyles to the point they are almost just a glamour.we have 19 jobs if you dont like one you can just switch to another one, its perfect to create jobs with different playstyles and difficulties.... not every person has to be able to play every job.
Most likely this shift is because we used to have 9 jobs to balance and now we have 19-20. By making 2 of each have the same playstyle, they become as easy to balance as it was in ARR.