To be honest, other than suggesting etiqueeteetiquette you've failed to provide a reason why a marked answer should be up voted.
An up vote is an indication that you think the answer has merit or was helpful. Marking it as the answer simply means that the answer helped you the most for your particular question. An answer can be correct and marked as the answer, but still be a terrible answer. There are also times where an OP will select their own answer - of course this cannot be up voted automatically.
Marking an answer and up voting an answer are two different things - don't try to morph them together.
Also remember that the policy on the site is that voting (up or down) is not forced or mandatory, and doesn't have to be justified. Automatic up votes would be at odds with that policy.