Keep in mind, unlike a bison, a LOT of cattle ranchers also use public pastures or rent out land (up here anyway). My grandpa has rented his land out for several years now for a herd of 20 cattle to come and calve during summer. Any doctoring has to be done with horse and rope, they aren't going to build doctoring chutes everywhere.
Likewise, the neighbours have a herd of bison. Due to the sheer size of a bison, they simply aren't mass produced like cattle are. There isn't the need to have to tag them, brand them, or routinely assist with birthing like you do with cows thanks to centuries of domesticated screwed up breeding. They are MUCH better equipped to still handle being left mostly to their own devices. And as stated, they're not a creature you want to transport much, so all bison owners I know keep them strictly at home where they have access to all equipment.