Simply because the surveyor is asking the question, respondents believe that they should have an opinion about it. For example, researchers have shown that large minorities would provide opinions on countries that don’t exist.
This applies to you too: you have fewer opinions than you express.
(source: <a href="http://www.overcomingbias.com/2011/07/making-up-opinions.html">http://www.overcomingbias.com/2011/07/making-up-opinions.html</a>)