Because as long as you study them correctly, you increase your piano-skill. Also, playing them not at full speed doesnt mean that you cant have fun playing it.gyzzzmo
But don't you find you tire of a piece after playing it for so long? Doesn't it get less fun after you've had it for a while?
If it's taking you months to finish a piece, why are you struggling to do it? Why not simply leave it alone and go back to it at a later date, when you're ready to?