Amongst the writers and other artists I talk to, there is frequent discussion of the muse. And while I learned the other day that there is in fact no muse for art (though ones for history and astronomy, which made the oversight seem even more strange) there are of course muses for literature and poetry and other such endeavors. I doubt any of us actually envision our muse as the classical version in the toga and sandals, but I have noticed that many do tend to envision their muse in a particular personification.
This may be a practice which is more prevalent among the poets I know. They seem more inclined to attribute their muse a specific appearance. Sometimes I think there's a blurry line between muse and inspiration, because there's been plenty of poetry out there inspired by specific people. Just look at Shakespeare's sonnets. I think the muse can inspire you with regards to having a certain person in mind, which also means at times, if you view your muse as an individual entity, it's probably going to take on the attributes of that inspiration on occasion.
I generally don't see my muse as a person, least not a specific person, but I find it interesting that the past few times I've actually been moved to write poetry, the muse has taken a definite form and personality. I'm not sure why, though I could speculate. And the poems aren't the only thing I have written where I've had a person in my head for inspiration - indeed, the same person who seems to be serving as my poetry muse has inspired some non-poetry pieces. Yet in those instances, this person doesn't seem to be acting specifically as muse, just merely as one of those sources I draw on in my head for my prose.
Maybe some of this has to do with the difference between poetry and prose, at least for me. Poetry has always felt more personal, more emotionally invested. As to why no one else has ever served as a muse before I can't answer. This isn't a scientific endeavor by any means, and as with any art form certain flights of fancy just are, in defiance of a concrete explanation. I could say something was different about this person - but each person we meet tends to be different, even if we form similar relationships with them.
So no, I don't know why, but I do know that now when people talk of their muse as being someone tangible to them, I can nod my head in agreement.
(Would just be nice if the muse would inspire better poetry from me, since that's the form the impulse seems to be taking in this muse's presence. As it is... well, there are reasons I don't write much poetry in the first place.)