Why? Thank God in Germany it is forbidden to requiring an account for online purchases and you can buy anything online without needing to register for an account first.
To be clear for online purchases I think it's a nice convenience to be able to buy without requiring an account, but for flights it's an entirely different game, you often need to provide a lot more PII.
The point is that I'm not forced to create and manage an account and I can do a second purchase without having to remember or reset a password to an account that was forced upon me.