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.
[0] I've been using vim-multiple-cursors for years, it's abandoned but still works ok most of the time.
Ctrl-V, then move down the lines you want to edit, Shift-I to insert text on multiple lines at once.