Even spaces that are easy to get into and replicate still have winners, household names to rule over all others (Dropbox, Github to name a couple of services that, ignoring scale, can be replicated pretty easily). At the seed stage, investors mark those winners in advance. At later round stage, investors back the de-facto winners (A16Z style). At some point the investment creates a momentum of its own, preventing viable competition and drawing in even more money (all the way to an IPO or acquisition).
The competition eventually dies and if the business case/segment has merit, the winner and their investors will stand to make all the money.
So Magic has just been marked as the winner in their segment (whatever you can call it). Whether or not this service is easy to replicate is irrelevant.