I have no evidence to back this up, but I'd be willing to guess that it's not about what jQuery does that vanilla can't do easily, so much as one of the following:
Supporting a legacy codebase
Not knowing that vanillaJS has feature {x}
Familiarity with JQuery - For example: If you had to choose between looking up how to do something in vanilla or already knowing how to do it in JQuery, which would you choose?
I have no evidence to back this up, but I'd be willing to guess that it's not about what jQuery does that vanilla can't do easily, so much as one of the following:
Can't argue with the above. Spot on!