There are several factors. First. The game engine is good. It is a custom version of Luminous Engine, but is not aiming for high quality graphics like the real Luminous. it is aimed for a MMO, a game which has to render hundreds of players on screen. You cannot do that if you try to have the same quality as Final Fantasy XV. It simply won't work, will require a insane computer specs even to run a low graphics.
And second, this game is aimed for a wide player base, with a very high variety on computers, from those with a i7, 16GB RAM and a RTX2080 to those which a laptop which uses a Nvidia mobile GT card.
And I think the second reason is probably the most important. If you vastly increase the quality and the hardware requirement ,you will cut your playerbase a lot. That's why is not good idea increase graphic quality. I think this game has been doing a very good job on this aspect, providing a very good optimization that makes the game run even on old laptops.
«But you can just simply buy a new PC!» It is not that simple as much people pretend. You cannot ask for the entire playerbase to change their computers just because yes, specially considering you have to pay each month for this game. That's why 32bits support has been for 6 years and only now they will stop updating it (not removing, as they said it will still works, but the new features may not work properly and causes issues).
To be fair, since release of Stormblood, I think they are neglecting a bit the optimization, such cases like Alphascape FPS problems. I hope are only exceptions and won't happen again on 4.5 and Shadowbringers.
It is simple, if you want more people to play the game, you need flexibility on graphic quality. The high hardware requirements are, the less players you will have. A bad idea for a MMO. And I think this game engine accomplish with that.