At least in Germany we have "sectoral bargaining" enshrined in law. The relevant union and the coalition of employers negotiate a deal (sometimes this process i

At least in Germany we have "sectoral bargaining" enshrined in law. The relevant union and the coalition of employers negotiate a deal (sometimes this process involves strike actions, if common ground can not be found initially). This deal automatically applies to all employees in the sector (i.e. metal workers). Some sectors do not have sectoral bargaining but certain unions have agreements with some subset of employers in that sector to have all their workers covered by any deal that is struck. The incentive for employers in those sectors (i.e. automotive) to join these agreements is that desirable employees usually prefer unionized shops, as they have better overall compensation packages and perks. Also they might not want to become the target of strikes. Not directly tied to unionization, workers in companies above a certain size have to be included in decision making (i.e. in public companies worker representatives get board seats) so a pro-union position often has a certain groundswell of support within a shop. For historic reasons, in some sectors union membership is part of the "culture", as well. Historically, unions have had an important role in upwards social mobility, popular education, culture, etc.