It looks like Facebook host a lot of ads themselves, so I guess that advertisers use URL shorteners as a way to verify click throughs from Facebook.
For unsophisticated advertisers (i.e. with no referrer log analysis) I guess it's pretty easy and effective.
In that case most advertisers are unsophisticated. Even the biggest ad-networks/retargeting/tracking solutions all depend on redirect via their servers to their end customer.
I am constantly disappointed with the technical knowledge and understanding display by our advertising partners. A large number of them have almost zero understand of how the internet work. Of cause part of the problem is that as a developer you're inclined to point out that something won't work because it won't work 100% of the time. That a useless answer for the advertising department and you end up with duct taped solution that works well enough, but of cause create their own set of problems.
I secretly suspect they -at least- know, but prefer to sweep these issue under the rug and that's why they prefer to involve as little technical people as possible. And that's why I don't work in this industry anymore and I won't come back to it until they've all burnt to the ground.
The fix is super simple: Their tracking pixel receive our order number, all they need to do is accept that it's unique and just tracking the first occurrence. Part of their problem is of cause that they switch to a new platform and didn't migrate any data.
Because many of the amount are so small, they just accept a ton of errors and as you say: "sweep these issues under the rug" because it's just a few cents so who cares. But it adds up.