And what is stopping them from having the placeholders a specific size, instead of expanding them when the javascript finally loads/starts up? If that sort of thing was unfeasible to do then fine, I'll give you your point. But they have no excuse. So they're either just too lazy to do it properly (and enjoy the nice mis-clicks on ads) or they just like the mis-clicks on ads and did it purposefully.
I'm not familiar with the systems, but I imagine that not knowing the eventual size of the ad would prohibit that. Or the fact that you'd have to have the user insert some HTML alongside your currently very simple <script> entry.
Of course, go back a year or so and everyone was screaming at ad providers for using blocking JavaScript.