First of all, I don't "know why", either.
I just tried it on my phone and can confirm it: in other apps I can
drag a "polyline" across the keyboard, and with dicionary-help it will
suggest/type reasonable words out of letters along the stroke.
With AndroWish, dragging over the keyboard just pops up tooltips of
the letters that I'm passing by.
Now for some related observations and musings:
The application must have some API or some flags for the keyboard app
as can be seen when an application has a numeric field and the keyboard
pops up in "number-mode". Maybe AndroWish also sets some flag upon
requesting the keyboard that for some reason inhibits the glide-feature.
The AndroWish has a command "borg" for most android-specifics, and there
is "borg keyboardinfo", which to my surprise shows "keyboard none hidden
0 hard_hidden 1" - despite the keyboard being there and visible.