Freak of nature mutation. Offspring of two species are usually infertile. It’s like how, in one family, you’ll usually have children with different heights, weights, and hair colours.

Giraffes are similar to other species because, essentially, in that area of the world, somewhere along the line, they diverged from a different species, which is the ancestor for deer and cattle (it looks like a horse or zebra, but is different, having cloven hooves and chewing its cud, and has a closer ancestor to them than to horses).