The canonical answer to this request is as follows: if you need multi-cursor (or, worse, multi-cursor with mouse support) then you are doing something non-Vim way (aka: wrong way) and there is a better way to do it.
If you need multi-cursor to do manual search and replace in text, then don't, just do automatic search and replace, maybe scoped to a block.
If you need multi-cursor for refactoring or renaming a variable across entire source file, then don't, use LSP plugin (or switch to Neovim) and do the proper refactoring action.
Sure, there are legit cases of using multi-cursor in Vim, but they are rare. So it's not worth to put it into Vim itself.
If you need multi-cursor to do manual search and replace in text, then don't, just do automatic search and replace, maybe scoped to a block. If you need multi-cursor for refactoring or renaming a variable across entire source file, then don't, use LSP plugin (or switch to Neovim) and do the proper refactoring action.
Sure, there are legit cases of using multi-cursor in Vim, but they are rare. So it's not worth to put it into Vim itself.