You don't seem to understand German work contracts well if you compare them to temp contracts.
The probation period also applies to you, the employee, if you choose that a company is not for you. Once that period is over, a three month notice period is quite common, so both you and the employer are locked into a relationship.
The probation period balances out how stable employment is once you both agreed that you're a match.
You don't seem to understand them. German law does not force you to have a three month notice period at the start of your employment, you can negotiate for a shorter period without the at-will part.
The probation period also applies to you, the employee, if you choose that a company is not for you. Once that period is over, a three month notice period is quite common, so both you and the employer are locked into a relationship.
The probation period balances out how stable employment is once you both agreed that you're a match.