I'm genuinely unsure, but I think we usually depict them as humanlike for a couple reasons.
>God became man, so humanity itself is inherently godlike. Depicting them as angels is a nod to this and honors the angels as well.
>Many famous angels take the form of men; the angels that are not strictly messengers like the Cherubim and Seraphim are in heaven praising God
>The nonhuman forms they take can be difficult to depict accurately
>Granting them a human form reminds us of their mutual shared nature as created and intelligible beings rather than as "thinking wheels" or what have you
The more I mull this over I'm reminded of the ban on depicting Christ as a lamb in icons. Biblically speaking there's nothing particularly upsetting about it, but it downplays the importance of the Incarnation in practice. I'm thinking this is a similar circumstance.