Ideally you want enough airflow from the intake fans to ensure positive pressure to prevent the GPU from intaking air from the rear, though most of the time yes, some air does end up getting pulled from the rear.

As for why, there are two reasons. One is there's been a push for more venting in cases to allow for better airflow and better temps. Most case manufacturers aren't going to be doing any serious airflow testing. At best they might do some temp testing, but many won't even be doing that. They're concerned with selling cases with features they think people want, and making it as inexpensively as possible.

And that brings us to the second reason. It's probably cheaper. You might think "but, that makes it more complicated to make!" And yes, it does, sort of. In both a solid or vented PCI slot cover is a punched aluminum part. There's something like the scariest cookie cutter you've ever seen that pops the parts out from a big sheet of metal. Any metal that doesn't get used is recycled back into new sheets. It also reduces the weight of the parts which saves money on shipping. So while the original die used to punch out the metal sheets might be more expensive to make, I would expect the cost savings from using less material and less weight add up to mean they save money overall.

It's like the old question of "why do toilet seats in public bathrooms have a slot cut out in the front?" People have lots of theories, but the reason is because it's cheaper because they use a little less material, and that's it.