Here I was thinking it was a Nigeria thing. But even people from our 51st state are getting the same treatment.
On a serious note, I don't know why such an inefficiency is allowed to exist. Prime opportunity for enterprising scammers and black markets to thrive.
You pay for you visa > you submit your proof pf payment with the traceable invoice number or whatever > they assign you a date when the next set of dates are released
Seems very simple to me but evidently it's rocket science.