Why do people use the first eleven chapters of Genesis for data on creation even tough it is one big poem pointing to our amazing creator?
Probably the best way to get an answer to your question is to ask the people who do this. They are in a position to tell you why they do.
I will agree with you that Genesis 1 is poetry. Most of Genesis 1-11 is NOT poetry. If someone told you that the entire first eleven chapters are poetry, then that person gave you false information. However, I am assuming that you are principally referring to Genesis 1:1 – Genesis 2:3. This section is poetry. I am not sure there is a rule that poetry cannot be taken to give factual information. There is nothing inherent to poetry which precludes if from including historical information. The fact that it is poetry is important. The fact that it is poetry ought to influence how we think about this passage and how we interpret it. However, the fact that it is poetry does not determine absolutely whether it does or does not contain factual, historical information.
In fact, there is an entire genre of biblical writing which is poetry and which is factual, historical information. An example is Psalm 78. Please do me a favor and read Psalm 78. Does this not prove that, at least in principle, biblical poetry might contain factual data about history?
Therefore, whether we like it or not, the fact that Genesis 1:1-2:2 is poetry does not determine absolutely for sure whether it includes historical information.
If you claim that this biblical material is principally conceptual and theological, I totally agree with you. It is also a polemic against the false theologies of Mesopotamian religion in the second millennium BC. Whether it includes historical information is something to be determined by a study of the material, not by any presupposition based on the fact that it is poetry.
So, I am giving you a somewhat vague answer on purpose, as I want to leave it in your hands to decide what you think, but I suggest you not judge people who take historical information in Genesis too harshly.