Right now, for instance, I have this page open in a web browser, which has youtube playing in one tab, twitter in another, and this in yet another. I also have an iPhone simulator running behind this browser, a terminal window tail -f ing a logfile, and vscode in another window.
All of this stuff is open at the same time. I can hear audio from all of it, access all of it, see all of it, all at the same time. This is not possible on an iPad.
Sure there are limitations; you can't hit F12 and drop into developer tools in Chrome, plus iOS Chrome is just reskinned Safari anyway. Oh and the sound thing. I'm not saying a full laptop doesn't have more, but the lost capabilities simply aren't showstoppers for everyone, especially if you're not a developer. In fact, because they can have build in cell-modems and macbook air's don't, combined with the fact that there are decent SSH clients, it's actually a better device for some.