I have a theory about why Veileds are considered more aggressive, but it's mostly hypothetical so please refute/add more information to this idea...
I think a lot of newer cham owners get Veileds from their local pet store as babies, myself included.
I read on the forums a while back (can't find the thread now, of course...) that chain pet stores get a bunch of eggs from the wild so they're Captive Hatched. These chameleons I have not noticed as being particularly friendly, including my own. I think this is because they're captive hatched, AND because I
imagine that being in a warehouse, they probably don't get raised with people in their face all the time. Key word here being imagine.
Meanwhile, most people who get a panther get them from a smaller breeding company, and have many generations of being handled especially from a young age.
This obviously does not apply to EVERY veiled or EVERY panther, but I think it's a factor in why we see Veileds as less "friendly".
I don't have a panther, but my carpet is from one of the best carpet breeders out there and he's very friendly (both the cham and the breeder

). We take little basking breaks and hang out together in my window almost every day (the cham - not the breeder).
However, I've heard from most people that carpets are one of the most shy species around. I think this is because carpet chameleons live shorter lives, there are less breeders, and this means a lot of them are WC or the first generation of a Captive Bred project.
Ok, I'm done rambling for now, if one of the more experienced keepers/breeders is reading this, please add! It's not a very developed theory lol.