The reason is that memory is not a unitary faculty and our memories are not the same as a mouse's reaction to shock. For example, the mechanisms by which we are able to recall Aunt Flo's wedding are not the same as those that enable us to tie our shoes or learn to avoid an electric fence. When most people think of a stereotypical 'memory', they consider highly conceptualized, detailed autobiographical memory from their own past. The fact that experimenters can break a behavioristically generated association in a mouse, doesn't shed light on our representational memory.
Strangely enough, Quine said something relevant to this in the opening pages of Word and Object. In this passage he argues that immediate experiences (e.g. sounds, electric shocks, pain) do not ground our language or our memories. We gauge meaning from our experiences by reference to physical objects, not the sense-data that we initially perceive.
...immediate experience simply will not, of itself, cohere as an autonomous domain. References to physical things are largely what hold [language] together. These references are not just inessential vestiges of the initially inter-subjective character of language, capable of being weeded out by devising an artificially subjective language for sense data. Rather they give us our main continuing access to past sense data themselves; for past sense data are mostly gone for good except as commemorated in physical posits. All we would have apart from posits and speculation are present sense data and present memories of past ones; and a memory trace of a sense datum is too meagre an affair to do much good. Actual memories mostly are traces not of past sensations but of past conceptualization or verbalization. Quine, W.V.O. (1960) Word and Object. MIT Press. p.2-3The point of this passage (for the sake of this post) is that the meaningfulness of our autobiographical memories is abstracted from direct experience or even recollections of sensation. No doubt our strongest memories are adorned with emotion, mental imagery and recreated sensations; but their cognitive impact can be abstracted from these feelings. Remembering is not simply retrieval; it requires the re-creation of past events. We furnish these re-creations with our imagination more than directly retrieving inputs. Thus a variety of non-memory-based mechanisms (e.g. current emotional state) impact how we perceive the past.
Because I believe that the mind supervenes on the brain, I have no doubt that neurochemical research will reveal much about our mental states. But, we're a long way from knowing how to selectively create a spotless mind.