More and more beginners nowadays do not want to install a whole new IDE. They don't want to have to configure it, learn it, and understand all of its nuances and foibles. They prefer using their existing setup (VSCode, Atom, vim, emacs, etc.) with a nice slot-in for the new language - which is exactly what the Haskell tooling supports.
By comparison, a custom IDE is a massive undertaking, and a relatively fruitless one. It takes a tremendous effort to do well, and up until that point it actually negatively impacts the learning experience: if you encourage beginners to use a mediocre IDE, they will experience the language in a mediocre way.