JQuery is often marketed as a quick way to build dynamic elements in your website, even if you aren't a JS developer to begin with. Many designers skip over raw JS because JQuery (and JQuery UI) comes pre-packaged with most of the common functionality you see on modern websites / web apps. It's a shortcut for a lot of people because it's simpler. The same way Bootstrap is often a shortcut for developers who don't want to work their way through CSS and responsive design, and would rather let a framework do it for them. Same things goes for Wordpress and PHP. I can crank out a dozen sites using Wordpress without knowing any PHP at all. In a sense, JQuery gives designers that same kind of "copy + paste" development ability that Wordpress does.
They're lose examples, but it's the nature of the industry. More and more people are being asked to overlap between disciplines, and the quickest way to turn out a product is to use something that gives you a leg up. Is it the right way to do things? Not necessarily, but as someone who started in JQuery before I back-tracked into raw JS, I can certainly say that sometimes the cart needs to come before the horse.
Overall, I do agree with you that in an ideal scenario, people ought to learn JS before JQuery. However, it's just not feasible for everyone.
Note: Sorry if this reads poorly. Running low on coffee this morning.