Sure, if you want to build it all yourself in JavaScript, but that's the point of a library isn't it?
And as far as "not targeting old browsers" there are plenty of places where modern browsers differ and jQuery normalizes their behavior. Chrome doesn't support mouseenter, Firefox doesn't support focusin, Safari's String.trim() doesn't trim zero-width spaces, their CSS uses prefixed properties, etc.
Also, talking about jQuery in isolation is like talking about C++ without its runtime library. One of the strengths of jQuery is its ecosystem such as plugins.
And as far as "not targeting old browsers" there are plenty of places where modern browsers differ and jQuery normalizes their behavior. Chrome doesn't support mouseenter, Firefox doesn't support focusin, Safari's String.trim() doesn't trim zero-width spaces, their CSS uses prefixed properties, etc.
Also, talking about jQuery in isolation is like talking about C++ without its runtime library. One of the strengths of jQuery is its ecosystem such as plugins.