Why cant the researcher conclude that people like the words more because they have seen them more often?

A researcher studying the mere exposure phenomenon (zajonc, 1980) wants to show that people like things more if they have seen them more often. he shows a group of participants a list of 20 words at an experimental session. one week late, the participants return for a second session in which they are randomly assigned to view either the same words again or a different set of 20 words that everyone had seen during the first session. the results show that the participants who have now seen the words twice like the words better than the group that only saw the words once, why cant the researcher conclude that people like the words more because they have seen them more often?