Why do you think so? Purism created its products using crowdfunding, i.e. they sold their devices (and free software on them) before anyting was created.
In the particular case of Purism Librem 5 phone, many people decided to buy the device in order to support the development of free software for mobile phones. As opposite to the PinePhone, whose developers do not develop any software and rely on the "community developers".
Really? If the car contained no software, people would still buy it over alternatives that contain software at various levels for a better driving experience, comfort, and entertainment?
Becoming employed by a car company that wants to improve the experience of their vehicle is a perfectly reasonable option though. Or writing software for someone else's car - there's more than enough gearheads who'd spend a ridiculous amount of money on custom code for their vehicle if only they were permitted to.