If somebody answers a question, he must believe that the question is meaningful and contributes to the community. Then why people hesitate to give other people credit for asking questions? I think if you answer a question, it makes sense to upvote the question for sure. Correct me if my logic is flawed...
You are correct, in the general case, and this has been brought up before. For example, this answer.
Unfortunately, people often get so focused on giving an answer that the value of and vote for the question can get lost. That sometimes happens to me when I answer, but I generally try to go back and correct the situation. It also helps if you edit the question or get the OP to edit the question to be more clear and include everything necessary in the question. You will also find that there are people who answer, but simply do not have the reputation to vote up, and there are people who do not really understand how it is all supposed to work.
As questions (answers, too) age, they tend to collect more up votes. The ideal situation is when a question makes the Hot List, then the question and answers to it get a lot of attention and votes in a very short span of time. We have had, so far, six Great Answer (answer score of 100 or more) badges awarded, and five of them took years to get that many up votes, but one of them hit the Hot List, and it got the badge in three days. There are only two Great Question (question score of 100 or more) badges, and both of those took a few years to earn the badges.
We also have several badges that can be earned by voting:
- Civic Duty Vote 300 or more times.
- Critic First down vote.
- Electorate Vote on 600 questions and 25% or more of total votes are on questions.
- Sportsmanship Up vote 100 answers on questions where an answer of yours has a positive score.
- Suffrage Use 30 votes in a day.
- Supporter First up vote.
- Vox Populi Use the maximum 40 votes in a day.
If you look at the all-time votes cast by user, you may notice that the advocate for voting in the answer I linked is not even in the top 10 for votes. It is simply one of those things that tends to lose focus when someone is working on an answer.
SE recently changed the reputation points for up votes on questions to be 10 (previously five) to match the reputation points for up votes on answers.