Surveys aren't to make you feel special, they're to gather information, so if I put a "both" option then guess what, 98% of you put "both", which shows nothing and is ultimately pointless. Everyone is "both" but everyone also believes it is either slightly more one or slightly more the other.
By giving you two options I force you to choose which one you are more. I am TJ enough to know that two options is optimal in this case