As far as I know, FFXIV is a 32-bit application which means that it's max addressable memory is limited to 4GB and in windows applications only 2GB of that 4GB is available for use by the application code. Once upon a time 4GB of ram was beyond HUGE, but now its nothing. I've run programs that can't even start with less than 6GB RAM and anything under 12GB would be awful performance.

Compare that 4GB to 17,000,000,00+ GB available (theoretically) to a 64 bit application and while no one I've ever met has anything close to that amount of RAM, but many people have 16GB and even 32GB of RAM on their home PC's. Porting everything from 32bit to 64bit would be a monumental task and may not actually result in a game that runs faster even though its no longer has RAM issues.