If you don’t remove the scrollbar, then the document can be scrolled underneath the modal. You can, these days, significantly mitigate this with overscroll-behavior on the modal backdrop, but the document scrollbar itself still works. (I will note, however, that overscroll-behavior is still missing on Safari. And scrollbar-gutter isn’t on Safari and is still disabled by default on Firefox.)

I think in a lot of cases, allowing the content below a modal to scroll is okay. If the user wants to scroll the content beneath a modal... so be it