Germans define realistic as understanding reality. And to them to understand reality, is to understand how that reality has become so, namely via its past. The present and the past are closely linked. You can't know the one, without knowing the other.
Americans define realistic as understanding the possible, to envision a future. Forward movement often demands moving away from the past. The present and the future are not closely always linked to the past, in some cases they are in opposition to each other.
From the German perspective, American visions are often not grounded or rooted in an accurate understanding of the status quo, of reality. They see Americans moving forward without first establishing their starting point and direction. Their orientation is not precise enough.
Too much emphasis on the present as a product of the past is seen as backward-looking. A vision of the future, forward movement, demands moving away from the past. To Americans the German logic seems to be "stuck in the past.".
Advice to Germans
Provide the historical context. But again, let your listeners know beforehand that you need to tell the full story. Your aim is for them to have a common understanding of the status quo before you can consider how best to move forward together. Talk history, but warn them first.
Advice to Americans
Try to hold back your natural tendency to jump from the present directly into the future. Take the time to explain the context of a given situation. This will necessarily lengthen the presentation. Do your homework and demonstrate that you have done it thoroughly.