There have been many times in the past where I've upvoted a reasonable answer to a question, but for one reason or another also downvoted the question. This can happen for many reasons. However, it would defeat the purpose of a downvote on the question if an upvote on a reasonable answer also carried an automatic upvote of the question.
It's important to bear in mind a couple things:
An upvote on an answer isn't a statement about the quality of the question - it's a statement about the quality of the answer. The two, while frequently related, are not actually directly linked; it's possible to write a good answer to a poor question, or the other way around.
The freedom to separately judge the quality of content is one of the integral aspects of Stack Exchange's voting system, and is one of the features provided that works very, very well for us here, setting us apart from many puzzle sites across the 'net.
One upvote carries +5 reputation - a downvote carries -2 reputation. In other words, every upvote counts for two and a half downvotes.
Generally, changes to the reputation system need to be thoroughly motivated. Reputation is integral enough that all changes will have unforeseeable consequences, and we're weighing the cost of those consequences against the benefit derived from the change.
One of the key parts of this is that you can't assume anything about what someone's vote means, beyond "quality content." In other words, the assumption that upvoting an answer means the question was interesting is unreliable.
You may, however, be interested in this question, which discusses higher reward for upvotes on questions.