"In either case my friends, who are on different servers, can't just click the link and immediately interact with the post like on other social networks."
They can do this in many of the mobile apps, a lot of them will automatically do the copy-paste thing behind the scenes. (For example Toot! does this, as do official apps.)
But this feature doesn't work all the time, and not everyone uses mobile apps, so the copy-paste thing is needed as a fallback.