[jQuery] find, not selector not working

[jQuery] find, not selector not working


I have the fairly simple markup below and am trying to only get only
the checkboxes under the first level LI (Main 1 and Main 2), not the
deeper ones.
<ul id="el">
<li>
        <input type="checkbox" value="1" /> - Main 1
        <ul>
            <li>
                <input type="checkbox" value="1" /> - Sub 1
            </li>
            <li>
                <input type="checkbox" value="1" /> - Sub 2
                <ul>
                    <li>
                        <input type="checkbox" value="1" /> - Sub 2 Sub 1
                    </li>
                </ul>
            </li>
        </ul>
    </li>
<li>
<input type="checkbox" value="1" /> - Main 2
</li>
</ul>
Using $("#el").find("li input") gets all 5 checkboxes. I've tried
multiple variations of find("li input").not("li ul input") as well as
using :not() in the selector which doesn't work.
One way that did finally work:
var notgroup = $("#el").find("li ul input");
$("#el").find("li input").not(notgroup)
Why does it work if there is a variable which is the result of the
selector, but not the selector itself?
Is there any better way to do this?