Due to the Briefmonopol laws.
No, not least because the monopoly on delivering letters fell several years ago.
In Germany there is something called Tarifautonomie which means that unions and employers are responsible for negotiating wages and politics is kept out. Unions and employers collectively decide what branches there are and terms and conditions are negotiated for entire branches on a state-by-state basis. Agreements usually contain opt-out clauses for individual companies as long as their works council's agree. The system has worked very well for years because it is efficient: individual companies do not have to devote resources to negotiating conditions and strike days are kept to a minimum. As a result German companies can afford to pay workers more than in other countries due to the increased productivity, as in the case from approx. 2000-2012, agree on pay increases below inflation to regain competitivity.
There are outliers with employers refusing to play by the rules using several techniques: cherry-picking or even setting up a compliant union with which to negotiate; sub-contracting to foreign companies who are exempt from the rules unless the federal government says otherwise (building work is not exempt, abattoirs are); or deciding that they belong to a different branch. Non-German companies often adopt confrontational positions until they understand how much easier and more profitable it is if they follow the rules.
We've also got a couple of smaller unions pulling an ASLEF: bringing a whole company to a halt even though they only represent a small section of employees. The German equivalent of ASLEF the GDL seems to be copying their militant tactics with Klaus Weselski every bit as militant as Bob Crow or Arthur Scargill. The pilots are doing the same. The end result, I fear, maybe a weakening or all employees' rights as well as bringing forward fully automated trains and planes.