OK, it’s not, and it’s not, but that’s not the question.
I understand the “long legs” part. I understand the “harvestman” aspect. What I’ve drawn a blank on is the “grandaddy” aspect? Why? According to OED, one of the other names is “grandfather graybeard.”
Two possibilities that I’ve thought of: A) the bent legs look sorta like walking canes, and B) they’re kinda grayish, so there’s the grandfatherly connection.
Any other ideas???
This question about entymology etymology is really bugging me.
Well, the point is moot here in Central Illinois, 'cause around these hyar parts it’s a “daddy long legs”, not a “granddaddy”.
Want me to do a Google search? 
I always figured it was just because it was the ‘daddy’ or the ‘grandaddy’ of all things with long legs. You know, proportionally, it has some pretty freakin’ long legs.
I always figured that it was originally “daddy,” and got corrupted to “grandaddy.”
Perhaps it has some reputation as the “old man of the forest?”
Actually, according to OED, it was originally “granddaddy,” and then got shortend to the more familiar “daddy,” which is the way I’ve mostly heard it, although I have heard older people when I was a yewt refer to them as “grandfather spiders.”
Another name it mentions for this critter is “Harry longlegs,” which is an interesting name, since (unlike a tarantula) it’s not a hairy spider. (I know, it’s not that, either.)
I would say that “daddy” and “granddaddy” were awarded to the spider because of its impressive size (thanks only to its legs). Granddaddy being a term of seniority, much the same way that Lions are kings of the jungle even though they don’t wear crowns or sit upon thrones.
If you can stomach the movie Wild Wild West, watch it and see if you don’t think that is one granddaddy of a spider.