Union contracts are employer specific (and in some cases, facility specific), and if the Tesla factory workers unionized, they
could negotiate for stock grants.
The threat was that Tesla would revoke stock option grants if the workforce unionized, or refuse to negotiate over the availability of stock options, which is illegal under current federal labor regulations.
No, most other automakers don't offer stock, because the UAW didn't negotiate that with other automakers. However, for most other automakers, (and indeed, outside of tech generally) stock options are not worth much because the stock doesn't appreciate rapidly and it's not worth the administrative burden of issuing/tracking options for share gains in the fractions of a dollar. Most other automakers do allow employees to purchase (actual) shares of company stock out of pre-tax income, which is generally a huge benefit for companies that actually make money and are valued on fundamentals.