Multiple cursors simulate not just macros, but also setting lots of marks and iterating the macros through them. It’s relatively easy to write an equivalent macro if you’re just placing a cursor every three lines or at every occurence of some string; you can have the macro move three lines down when it’s done, or search for the next occurence of some regex. But if you’re actually clicking on places in the code that only a human can identify need changing, then it’s much harder to write an equivalent macro.