Part of it is because some are not "just" bunion surgeries (which are bad enough in themselves!)

With my recent surgery, for example, it was not done primarily to remove bunion - my 1st metarsal had splayed out so much, and was shortened (from previous bunion surgery in 1980), along with a too-long 2nd metatarsal, so that my "big-toe joint" wasn't bearing the weight as it should have been - 2nd and 3rd metatarsals were bearing weight instead, causing painful stress fractures in 2nd metatarsal, as well as severe metatarsalgia throughout 2nd, 3rd, & 4th metatarsals. The bunion was big, but wasn't really causing much pain, and was not the primary reason for the surgery I just had... The foot had to be fixed mechanically so it would bear weight properly, and to do that, my pod had to cut and re-align both the first and 2nd metatarsals (shortened the 2nd one) and add pins to (hopefully!) keep everything in place so the 1st metatarsal will function as the weight-bearing joint/bone that it *should* be. My doc also had to additional straightening of the 1st metatarsal, with an additional pin, and shave off the bunion (comparatively the most minor part of my surgery), as well as remove some cartilage that was damaged by arthritis... SOOoooo... with lots of bones being cut, and more invasive stuff being done, there's more the foot to be need for non-weight bearing, so it can heal properly.
When I had surgery at 19 and 20, it was bunionectomies with Chevron osteotomy, so the 1st metatarsal was cut then too, but in a less destructive way (can't think of a better word) and not moved nearly as much as this time, so I was able to be weight-bearing, with a walking-cast, by the end of the first week. I WISH it was that way this time too!

Unfortunately, that surgery was not a permanent "fix" as the 1st metatarsal still splayed out again - 20 years later though, so it did work for a good long while.
There are SOoo many different procedures, each for different problems (not all bunions are "simple bunions" - many have other, more complex problems going on as well), as well as different doctors, who all have their own way of doing things, and all have different recommendations for recovery (WB vs. NWB, how long - 2 wks. vs. 8 wks - pins removed or not, etc. etc. etc.) there ends up being all kinds of different recoveries.
JD