Why aren't vertical dropouts truly vertical? Making some custom rear dropouts (vertical) for a project (nonstandard bike, they must be custom) and as I look to dropout examples, I notice that "vertical" dropouts never seem to have the slots completely vertical, as in 90 degrees from horizontal. Is there a reason for this? My design keeps the slots perfectly vertical for ease of drafting, cutting, etc.
Likely to clear the derailleur.  Angling the dropouts somewhat gives more clearance between the derailleur and the cassette as the wheel is being removed or installed.

While the derailleur could also be moved more rearward and upward to provide the same clearance, that would reduce the length of chain that engages the sprockets.