Of course, you can implement these things yourself in Javascript, but why should you do that? Are you sure that copy-pasted code you found on the internet really works correctly on all browsers, or was it an 80% solution that got lots of upvotes on StackOverflow by people that didn't test it very thoroughly? Do you want to have to learn a hundred different browser gotchas the hard way, and is that really a good use of your time?
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There are still quite a lot of things that jQuery provides that are not one-liners in Vanilla JS. For example - serialize, and event delegation with on. And then dozens of other bug fixes and conveniences.
Of course, you can implement these things yourself in Javascript, but why should you do that? Are you sure that copy-pasted code you found on the internet really works correctly on all browsers, or was it an 80% solution that got lots of upvotes on StackOverflow by people that didn't test it very thoroughly? Do you want to have to learn a hundred different browser gotchas the hard way, and is that really a good use of your time?