Simply because it's easy to vote somebody down without giving a reason, leading them to not know what they're doing wrong and continue behaving in the way that got them voted down in the first place. If a user's doing something wrong, it should be addressed more clearly.
You're misunderstanding that it is a superfluous system with no real bearing as to 'what you're doing wrong'.
To elaborate:
1. "It's easy to vote someone down without a reason" - but it's just as easy to vote someone up without a reason (though usually the reasons for both of these are the agreeability of said post).
2. "leading them to not know what they're doing wrong" - if you are doing something really wrong, then you WILL be notified by a moderator or an administrator. The "respect" system has nothing to do with what you're doing that may be breaking the rules of the forum (which I'm assuming you've read, and if you have, you would know what you're doing wrong and right)
(The rules thread can be found
here)
3. IF you find your posts being downvoted, it's likely that you're just saying disagreeable things... though admittedly, there have been instances in which people's posts have been downvoted en masse. Again, this has nothing to do with what you're doing right or wrong with regards to forum behavior that may get you warned/banned.
I hope this information helps you.