It's easier to run LCD production lines if all the panels are produced on the line at the same aspect ratio. Given the prevalence of 16:9 video panels, this made it easier to produce 16:9 panels for laptops. (So, you're essentially correct.)
I'd expect laptop screens, having a different pixel density, to be manufactured on different lines so that having the same aspect ratio as some other size wouldn't count that much.
If the Pixel density differs ( a lot ) then yes it doesn't matter. But you still need economy of scale. This doesn't matter as much now because current LCD panel industry is essentially left over by Smartphone market. So you get a lot more flexibility than trying to print millions of the same 16:9 panels.
