Not as cheap and easy as calling in a fake bomb threat. I Googled it and found a surprising number of articles about different incidents where airports were shut down this way... in several different cases, the articles identified the culprit as someone who was late to their flight and wanted to delay it! But that's a spur-of-the-moment decision, whereas bringing a drone requires planning. And other than that, what kind of motivation is there to disrupt an airport for disruption's sake? There's terrorism, but terrorists typically want to produce a body count, not just inconvenience.

And I'm surprised that major airports don't have counter-measures.

The vulnerability is huge, and not only for airports.

There's a pot of gold waiting for the right product.