There are more citations per article in Wikipedia than in Brittanica. Is Brittanica an unreliable source?
We know who wrote Britannica. We can (or at least, should be able to - let's not get into a discussion about the actual reliability of Britannica) trust that their authors really do know what they're talking about, and even if they are writing from memory, their memory is more reliable than that of a random Wikipedia user we know nothing about. (Not necessarily more accurate, but it is more reliable.)