, but they seem like they would serve a pretty similar function in the ecosystem

They do not graze the same and these differences hurt ecosystems

Cows and bison differ in all behaviors — including how they stand, move and graze. Cattle spend almost double the time grazing — 45-49 percent — in comparison to bison — who spend just over a quarter of their time doing so (26-28 percent). And while land needs some amount of disturbance to enable soil turnover, it turns out that the kind of soil disturbance provided by cows trampling over the soil is not the best for preserving biodiversity. And these critical differences in grazing behavior have downstream effects on the ecosystems they inhabit.

[…]

The types of plants bison and cattle prefer to graze on are also remarkably different. Unlike cattle, who prefer to graze steadily on flowering herbs like clover, milkweed and sunflower, free-roaming bison cover a much larger area, grazing on different varieties of grass, such as perennial grasses, but leaving certain areas of the prairie untouched.

https://sentientmedia.org/cattle-ranching-terrible-for-biodiversity/

Which leads to:

Livestock farmers often claim that their grazing systems “mimic nature”. If so, the mimicry is a crude caricature. A review of evidence from over 100 studies found that when livestock are removed from the land, the abundance and diversity of almost all groups of wild animals increases

https://www.theguardian.com/environment/2022/aug/16/most-damaging-farm-products-organic-pasture-fed-beef-lamb