75% (in 16:9 AR) is used by a lot of older games (and some new), so by default this will be what many players are used to. The reason for it being 75% is because older games use FOV based on 4:3 resolution (like CSGO), and the monitor match is 100% for this aspect ratio. When you play these games on a 16:9 monitor the match remains at the 4:3 portion of the monitor, meaning 75% for 16:9 ((4/3)/(16/9)).
Newer games mostly use 100% (like all Unreal Engine 4 games), while some use 0% (i.e. Call of Duty). So 75% is a good default value in between.