The glyphs can look completely different in italic. http://www.fffranziska.com/#sec_italics And after drawing the glyphs, manually checking and adjusting the kerning between every pair of glyphs takes a lot of time too.
You are right. From reading the comments on the issue [2], this seems to be the reason why it takes so long. Although I have now idea of how fonts are made, I think the work needs to be done for all currently available weights (7 different weights).
So the time needed is probably shorter than the time spent on the regular variant, but only by a small factor, certainly not orders of magnitude.