I think you're right in that 'he's not good at throwing them'... but 'can't' is a stretch I think... not that you meant it that way - MY point being... I think it's odd that, (and only IF this is true), they never even tried even with the limitations.
At the combine the QB's were throwing a 3 route, (sorry, I'm old, not sure what that route is called these days), and Wentz and Goff were simply light years better than the rest of the group of which one, as you know, is doing quite well right now.
Mannion threw a couple skinny posts in preseason and absolutely NAILED them... the two I remember ( could have been more), were jaw dropping accurate even though one, (if memory serves) was dropped. - JamesJM