Maybe because that way it just might incite people to read all the nodes in a thread?

If you don't have too many nodelets displayed, for any reasonable active thread your favourite browser should have a keyboard shortcut that jump to end of page (ctrl-end in Mozilla), and there you'll find the vote button.

Thinking about this a bit more, my HTML fu isn't brilliant, but I'm pretty sure you can associate a keypress with a specific form element. Something like ctrl-shift-v would move the focus to the vote button.

In fact, all the voting widgets should be at the bottom: the voting radio buttons should be at the end of the node, not the beginning, because it's only after you've read a node that you can decide which way, if any, you want to vote. But I take no credit for this observation, many people have noted this discrepancy.