That's not at all the reason.

Encyclopedias are tertiary sources, compilations of information generated by others. They are neither sources of first hand information (primary sources) nor original analysis (secondary sources). You can't cite encyclopedias because there's nothing to cite. The encyclopedia was not the first place the claim was made, even if it was the first place you happened to read it. You don't attribute a Wayne Gretsky quote to Michael Scott no matter how clearly he told you Wayne Gretsky said it.