Mainly because so much jQuery already out there. For a fresh project it's harder to make an argument for it, but if you work with legacy code you probably have to know jQuery. Personally I like jQuery. Like so many developers I started using it a long time ago because it solved numerous browser inconsistencies and incompatibilities. Then I got used to the clean and concise syntax. Now I use htmx more than jQuery.
For fresh project, maybe it's better to use a modern tool
You may not need jQuery because vanilla JS can do the same things now. Or you may have other tools that do what you need. But choosing something because it's new or supposedly "modern" only makes sense if the alternatives are obsolete or prevent you from doing something, and jQuery isn't either.