It’s probably a good thing that airline companies don’t design their own planes cause we’d probably end up with a lot of fuselage-centric designs. But not doing design work like that doesn’t mean there aren’t opportunities for academically trained hardware engineers.
In satellites, we bought a lot of hardware from other aerospace companies but we still assigned some our engineers to be product experts on the items we bought and integrated. There were multiple reasons for doing so: We had in-house expertise to do the first level troubleshooting when things went wrong, we had people who could weigh in on recommendations from the subcontractor when they provided potential causes of issues, fixes, or improvements, and we had people with the correct expertise to evaluate proposals from multiple companies when it came time to build new end items to name a few. I imagine airlines have some in-house experts for some of the same reasons. When there’s an issue, you don’t want to blindly accept what a subcontractor says and unless you have an engineer who is up to speed on the item, you’re stuck in that position. There just isn’t time to get past the learning curve; time is money and waiting around just unacceptable.
You won’t find airline companies designing their own planes. They also won’t design and test key subsystem hardware. If that’s your interest, go to companies that supply the hardware. But if you don’t mind being in an oversight role, there could be a job for you.