What Jetbrains provides, and is quite hard to replicate, is the linting and auto-correct suite for medium/large/enterprise development teams.
That's actually the main thing, deep and wide integrated linting.
Almost everyone does autocompletion (up to a point).
Almost no one does linting as well as they do.
Quick experiment for you.
Take a big source code file you've worked on, preferably solo, so you won't have excuses :-p Not a "best case scenario" file you've cleaned up and polished. Be honest with yourself, something big and reasonably messy where the deadline was tight or the issue was complex.
Open the entire project and configure it in a Jetbrains product.
Open that specific file in the IDE.
My guess is that the top-right green checkmark won't be there.
You just can't hold all that stuff in your head at all times.
That's why we use computers and tools (such as advanced IDEs), to remind us of this stuff.
Anyway, in the rare case that you actually do all of that on your own, <<you>> are an outlier. 1%, 10%, for sure you're not lower than 50% of developers. The majority of developers need that help.