Well sometimes wikipedia is not as bad as you would think... you could specify though that you already know about the wikipedia article about your question and that you look for more in depth information... maybe this keeps the wikipedia citation from coming...
Because if you google anything, wikipedia will show up pretty high up in the hit list.
Besides, wikipedia is a great source for information, and if you do not trust it (as a lot of people do, usually those who do not contribute) you might want to check the references at the bottom of each page. There you might find a source more "trustworthy" on the subject.
Although Wikipedia is always reliable, I think the question aims at getting people to answer based on their own experiences. If they wanted the Wikipedia answer, they could find it themselves without bothering to formulate a question.
Why people answer your question with a quotation from wikipedia?
I find wikipedia can provide general information / definition about almost everything but not necessarily accurate. Suggesting to use wikipedia as rough understanding of something & perform further research or google search other supporting websites / artifacts to validate / strengthen your understanding.
Second reason might be some people prefer to start from basic & also to use wikipedia to verify / validate their own understanding first before answering any RG question (to ensure correct answer is given).
Why people answer your question with a quotation from wikipedia?
If I can interpret above question from different angle - it might be asking why RG members eagerly to provide answers from wikipedia instead of providing answer based on their deep expertise. Because searching answer from wikipedia can also be handled easily by the RG member that posed the question.
Think different people with different background might interpret a question differently. So general question might attract general / various answers and specific question might attract specific answer. Perhaps during asking a RG question, we can specify explicitly asking answerer not to use wikipedia / typical Internet answer or the questioner proactively put all the wikipedia / typical internet answers in the RG question so that answerer need to provide their answers based on their deep expertise / deep understanding on the particular issue.
An interesting question but i think that wikipedia is not really a reliable scientific source of information because every one can edit the text in this website. May be some efforts are required when it comes to reliable and valid answers that will enrich the debate about a particular issue.