Specifically, in unionized environments introducing new technologies and methods no matter how seemingly trivial creates an obligation for formal job training. Setting up an hour of training for 5k+ people on how to install and use Auth0 is way more expensive than just sending someone an SMS. There is no implicit responsibility to adapt to change.

If an employer wants employees in a bargaining unit to use their own mobile devices and install a TOTP app on them, employer has to pay, and then the responsibility for it working needs to be established. In normal environments, you just say "we use this here," and users figure it out. If it's organic bottom-up adoption based on the option, it works, but if you impose a change, it creates admin overheads.

I am not sure which countries you relate to, but requiring pepole in France to use their private phone for work is simply impossible.

That's too broad a generalization. I am aware of unionized environments where the employer uses MFA and expects the employee to bring their own device, and the user is expected to read the online KB documentation in lieu of formal training.

tl;dr -- unions aren't all alike, and don't all bargain over the same issues.