Why would Barr, Saussure, and others think that speakers and writers have only the present meaning of a word in mind? Does it perhaps have something to do with the fact that they have only the present sense in mind?

As the previous post showed, this is hardly a universal prejudice. The decline of interest in etymology is fairly recent (cf. Yakov Malkiel’s Etymology for a history of 19th and 20th centuries).

Have we all perhaps been trained to ignore word origins and historical meanings? Just because we’ve been trained in this ahistorical frame of mind, why would we think that the minds of ancients, medievals, or biblical writers ran along the same tracks?