jQuery UI $(...).sortable is not a function with WebPack

I believe I've set everything up correctly, but I'm getting an odd issue with Webpack.

Consider this simple app.ts file:

'use strict';

import $ = require('jquery');
import 'jquery-ui';

$(function() {
    $( "#sortable" ).sortable();
});

Everything compiles fine, but when the site is run it complains that the Uncaught TypeError: $(...).sortable is not a function. (sortable is a jQuery UI function).

Everything works fine when I instead link to a CDN hosted version of jQuery and jQuery UI, but it doesn't work when I use JS modules and Webpack. Why is this?

Why is the jQueryUI function sortable() not recognized?