Why are indefinite singular third person pronouns treated as plurals?

It's always bugged me.
My research behind this question was rather shallow, so I apologize if it's a duplicates.

Thanks!

Edit, example:
Say you don't know who you are talking to on Reddit, you simply know -their- username. Is it just the way in English to call people when you don't know who -they- are? Thanks again!

Answer

In English there is no third person singular gender-neutral pronoun†. We must say "his or her", and so many people say "their" instead -- so many, in fact, that "their" should be entering the grammar and style books as a viable form, if it hasn't done so already.

†i.e. no marked form.