* edit: What meant say is that Amazon must offer a job that's better than the jobs that people currently have for them to switch.
These agreements are not so much voluntary of course, since without an agreement in place it's hard to do any business in the country at all. Some try out of stubbornness but it's just an expensive way to accomplish nothing.
The alternative would be meddling politicians and nobody wants that. Unions are expected to have a bit more domain knowledge, and these agreements are re-negotiated every few years to get rid of obsolete rules and introduce new ones.
It helps that the labor unions are centralized, professionalized, and has vested interests in the long term survival of the companies they regulate, because of the trust required to make it work. If it is a good system I do not know, but it does provide for a stable labor market with almost no strikes, for which an agreement is a small price to pay.
(As a side note the current government is actually just about to abolish one of the corner stones of those laws (“LAS”, the law of employment protection). Due to a deal they cut with the opposition parties in order to be allowed to form government)