I prefer "nee" - it's specific.
While, others may like to use "maiden name", also specific.
And even some will use "formerly" which means before, and can be used in a wide range of discussion, not specific to "nee" or "maiden name" or "birthname".
Guess that here in WikiTree, its simply a matter of who has designed the site has chosen and gone with "birthname" - also specific, just more letters are used to convey the same meaning as "nee".
All part of having an extensive vocabulary.