It's because these tools are used to deliver high quality physical products on time and on budget with no scope or interest in messing around with software defects. These tools are treated like physical tools. It's worth paying for a high quality one which won't cause unexpected grief, by people who will provide immediate support because we pay them.

I think if there were good quality open source equivalents they would be considered, but they pose a huge risk, possibly even an existential risk, if they derail our development plans unexpectedly. Paying a lot of money for seriously good quality tools reduces that risk dramatically.

I've had a brief look at FreeCAD, and it's got a lot of potential. But when you compare it with SolidWorks, OnShape or SolidEdge, there's clearly a huge gap in usability and capability which needs closing before a lot of people will be able to consider it seriously. I'm sure it will eventually get there, like KiCAD did, but it will take many years and a lot of investment to get the usability, polish and featureset up to parity. It looks like Ondsel did a really good job to make some progress along that path.