Is it actually required that an employer “come to the table” at all? Presumably they could say no, and if employees don’t like it they go on strike, but then wouldn’t they all be let go anyway?
Yes, and then the employer has no employees, no business and no profits. Employers tend not to want to drive themselves out of business, which is why they come to the table. Not because they choose to out of a sense of personal generosity, but because employees acting as a collective have leverage over management.
Also, in the US, employers are required under the National Labor Relations Act to bargain in good faith with union membership. It is also illegal to fire employees for organizing into a union.