It's called Job Fantasy. Mages in most fantasy games, table top or otherwise, tend to have the "wisend sage with flowing robes" trope. Hence why every mage job in this game, and most other Final Fantasy titles that have a majority fantasy theme, i.e. less modern aesthetic(I'm looking at you bikins and blue jeans >.>), tend to wear robes of some sort. The mage jobs in this game each wear the "robe" aesthetic differently. I'm honestly surprised you decided to call out Summoners designs vs. White Mage which has literally had some of their Artifact Gears look like something a Nun would wear. Regardless, the wonderful thing about Final Fantasy XIV is that is has a glamour system so if you don't like the way something looks you can change it.