I've done a couple of projects with engineering companies including one at a power plant. From what I've seen, the thing that tends to lead from air gapping to lack of airgapping is support.
The engineering companies don't have the IT infrastructure experience or skills in their engineering practice. They hired me to do basic stuff like SAN setup, switch configuration, VMware, etc.
The engineering company is required to provide support for their subsystem for a period of a couple of years and this includes everything IT related. Their office is hundreds of miles from the plant so problems with the IT environment require them to fly someone out. This is expensive, the guy who goes out has limited troubleshooting and they turn to me.
But they don't want to pay for my services on site, so ultimately they end up ungapping the environment so it can be supported with less cost. They have some security -- VPN only and possibly other restrictions which limit VPN connectivity, but they break the air gap.
They could maintain the air gap, but it would cost money -- support and travel costs, etc.
Ideally the engineering company would make IT systems part of their practice, but I think a lot of engineers have an "I'm an engineer" mentality which makes them they're good at everything, so they see this as unnecessary. They could negotiate with the plant to engage their IT resources, but that would cost them money.