Question

Why does APA not allow authors' first and middle names to be used in content, citations, and references?

Answer 1

Last name only is used in citations except for personal communications citations inwhich case you use first name and last initial.
Last name and first and middle initials are used in references except for personal communications, which do not requires a reference page entry at all.
Last name OR first and last name and be used within your content.

As far is why this is, it's hard to say for sure because the APA manual does not mention the reasoning for this, but if I had to make an educated guess I would say because it is not necessary. Your reference page will poin the reader to the source of your information where they can find the full name of the author if they wish. First and middle names are not entirely necessary in light of readily available information. Knowing the first or middle name of the author is not going to add any valuable information to your paper, so initials suffice to just distinguish authors with the same last name from one another.