because the feat does more than just give you an advantage on checks within your field of study
the one field of study you choose is your “primary” field of study.
There are times that a field of study will automatically give you information (you will see this applied in the blessed lands book) additionally when a bit of lore falls within one of your fields of study the DC required should be lowered. lastly, there are times that lore will become available that unless you have the applicable field of study you don’t even get a roll to recall those facts.
the feat is a lot more than just advantage.
