Everyone knows it is; or I should say, if you've tried it then you know that. Let me start again: Trust me, FaceTime is weird and awkward. Seems like a good idea, being able to see whoever it is you're talking to - although from what I have read on The Internet, it's mainly good for showing the person on the other end what their idiot children or ugly pets or your malformed genitals look like, since they're across the country or ocean or whatever and their memory ain't work for shit.
The only real article I found (in an exhaustive 8-second Google search) that addresses that FaceTime might be awkward is this one, a sort of point-counterpoint. The first guy thinks that it's awkward mainly because you're used to doing other things while you talk on the phone with someone, either idly or actively, and you're suddenly required to give the person your rapt attention. A valid point, but it doesn't explain why it isn't uncomfortable to have a conversation in person, when you aren't free to browse online or examine your cuticles or unravel the drapes without seeming inattentive. (The other guy in that article disagrees, but then he starts making dumb jokes and I stopped paying attention to his argument.)
There are actually a lot of these, Q & A sessions about what your conversation is literally supposed to consist of over FaceTime. They're hilarious.
My short point is this: FaceTime is awkward and weird because you're FACING the person while you talk to them. It's essentially as if you're standing at arm's length from them and speaking directly into their face. They're watching your mouth, you're watching their eyes - there's nothing else to look at.
Allow me to digress into my own personal semi-coherent observations of human interactions and body language, and wonder if you've ever noticed that you almost never actually Face someone, looking directly At their Face (even from like five feet away or a normal conversational distance) when you talk to them casually. With dogs, facing and making eye contact is perceived as a threat; for people, it's usually reserved for situations like when you first meet someone, or for like when your boss is yelling at you for stealing or lying or whatever (again), or for when you and a prospective sexual partner are both drunk and trying to get in each other's underwear at the end of the night, which I think is frankly disgusting.
If you want to test this theory, simply stand square-on, face-to-face with a person the next time you're having a casual conversation with them. It's very hard to maintain for any period of time. If you insist on continuing to Face them, they may turn sideways, or subtly start almost walking behind you. Or if they're a dog, they might bite you.
Anyway, that's why it's awkward. The end