It kind of is, though. Grafting specimens of the same species is fairly easy - grafting onto a different species but staying in the same genus is a little less

It kind of is, though. Grafting specimens of the same species is fairly easy - grafting onto a different species but staying in the same genus is a little less reliable but the basis of most grafted orchard trees. Grafting to a different genus, but within the same family, is much more challenging. Not impossible, of course, but less likely to succeed, so less commonly done.