Why?
Are they that beamy?
In a word, yes.
I hadn't really thought much about it, since it's been an intuitive thing for me since my first planars (MMGs). Something about aimin' 'em right at me has always appealed.
After some thought, I'd have to agree that their beamy nature is the reason. Being in the center is always best. At the Sanders room in RMAF 2016, there were only three listening chairs, lined up behind each other, each in the center. But I don't want to give the impression that it is required. Standing up, for instance, brings an angled perspective on the hologram, making depth perception and localization more pronounced. The 'sweet spot' standing up allows two people to share it, IME. Seated, two people had best be cuddly.
I can tell you that a friend preferred them with less toe-in, but I went back to more after a day or so. It seems pretty good for next-room listening too, not sure why. In my experience, planars belong in different, intersecting planes.
Edit: On second thought, being in the center may be as much about being equi-distant as about dispersion.