If they did they'd bring Tesla to the negotiating table in an instant because there would be no way for Tesla to continue doing business otherwise. They'd have a choice between negotiate with the unions or have empty factories with no workers, which is no choice at all.
How well the model works (or doesn't) has no bearing on the enthusiasm for striking at a particular workplace.
As I wrote in my comment, it is not surprising at all that there is meagre participation from Tesla employees. The conflict is not between the employees and their managers. The conflict is between the Tesla enterprise and a handful of national labour organizations.
(There could very well be tangible upsides for Tesla employees, if the unions succeed. Or maybe none. It is not really relevant and I am not familiar with the details).
Strikes isn't organized as a per company basis, so this doesn't make any sense at all.