(From the outside) it seems hard to understand why something should be effectively be mandatory without being officially mandatory.
If collective agreements are to be mandatory, the government should just pass a law making them mandatory, then that's done.
Hiding behind the claim "it's up to each employer and their employees" to decide is just silly.
Laws are never perfect, and writing one just for the sake of it both makes complying with the law more complex and opens opportunities for people with money to create legal moats around interpretations and case law precedence.
Well, I'll see that and raise you:
If remuneration packages at Tesla (including in Scandinavia) "pretty much works" (in the sense that Tesla are able to attract and retain talent, which they apparently are), why write a collective bargaining agreement?
Tesla wants to punish anyone who joins a union which is super illegal. We're talking prison time if they don't back down. They can't take away the stock benefit when IF Metall wins. They've put it out there, and if they take it away because of unionisation, they'll lose. It's a huge fucking no.