Since we never seem to have a viable third party candidate, we end up choosing between two people. A vote for one is automatically a vote against the other whether we meant it that way or not.
Both the Democrats and Republicans have a history of running geriatric disasters, not because they are the best candidate, but because it's "their turn." When a candidate is old enough and/or appears to be mentally ossified, people are forced to take that into consideration. Bob Dole lost even here in my Republican state, mostly because he appeared old and infirm.
I do understand what you're saying though. Candidates who truly inspire their voters are few and far between.