Because XIV is build upon a crap code and an even more crappy engine from 1.0 with some tweaks. The job also had to be done within 2 years or so while maintaining and patching 1.0 at the same time. It is an industry miracle which happens for first time in this category. They never expected this success. They took a desperate measure to save what they could and they needed to do it fast. ARR was created. It carried some crap code from 1.0 thus the limitations. Basically some things CAN BE done but they choose to let them aside because they will need more work force and time and they can't sacrifice development time and resources from the patches and the playable content.
I just wish to see the game go forward and the graphic engine to be enhanced and improved. It's really silly we have almost NO character customization in 2017. 4 faces really? Like it's a game from 2004 or so.
It definitely needs improvement.