Why do we feel tired if we sleep excessively?
Studies have proven somewhat the opposite of what you're asking



For example a study from 2004 - "The impact of extended sleep on daytime alertness, vigilance, and mood" found that,


  Average daily POMS vigor and fatigue scores also improved during the sleep extension period


And thus concluded


  Extended sleep leads to substantial improvements in daytime alertness, reaction time, and mood.




Whilst an older study from '73 -"Performance and Mood Following Variations in the Length and Timing of Sleep" found that,


  Neither sleep duration, nor any other sleep parameter consistently correlated with the performance or mood measures.


The study does go on to state however,


  Therefore, it seems reasonable to assume that the detrimental effects of acute changes in the length and/or timing of sleep were not directly related to any specific changes in the length or electrophysiological patterns of sleep, but to as yet unknown physiological changes resulting from the disruption of an established circadian rhythm of the sleep and wakefulness.


This perhaps suggests that the question which should be asked should not be why does oversleeping makes us feel tired, but why does the disruption of normal sleeping patterns make us feel tired?