Voting is not a structured intellectual exercise like taking a test or writing a paper. There's not a single "right" answer that you have to be smart to figure out.
Voting is an exercise in representation. People vote based on what they want, not what they know. Desires, dreams, and concerns are not knowledge. Smart people, educated people, still have emotions, can still have hugely different values, and want wildly different things.
The purpose of democracy is to adjudicate between competing desires without violence. If you try to exclude a category of people from this process, you harm its legitimacy and it stops working well for everyone. The end point of that trajectory is revolution.